hgweb: drop references to deprecated cgitb

cgitb is going away and gives warnings when importing, and that make tests
fail:
  $TESTTMP/hgweb.cgi:5: DeprecationWarning: 'cgitb' is deprecated and slated for removal in Python 3.13

The lack of a "nice" high level error handler is not a huge problem, neither
for users (where it is disabled anyway) or for tests (where we don't use a
browser and the plain tracebacks often are more readable). It is inevitable
that it is going away, and there is no obvious alternative. Remove it and move
on.

contrib/hgweb.fcgi

@@ -9,9 +9,6 b' config = b"/path/to/repo/or/config"'
 # (consult "installed modules" path from 'hg debuginstall'):
 # import sys; sys.path.insert(0, "/path/to/python/lib")
 
-# Uncomment to send python tracebacks to the browser if an error occurs:
-# import cgitb; cgitb.enable()
-
 from mercurial import demandimport
 
 demandimport.enable()

contrib/hgweb.wsgi

@@ -8,9 +8,6 b' config = b"/path/to/repo/or/config"'
 # (consult "installed modules" path from 'hg debuginstall'):
 #import sys; sys.path.insert(0, "/path/to/python/lib")
 
-# Uncomment to send python tracebacks to the browser if an error occurs:
-#import cgitb; cgitb.enable()
-
 # enable demandloading to reduce startup time
 from mercurial import demandimport; demandimport.enable()
 

hgweb.cgi

@@ -10,9 +10,6 b' config = b"/path/to/repo/or/config"'
 # (consult "installed modules" path from 'hg debuginstall'):
 # import sys; sys.path.insert(0, "/path/to/python/lib")
 
-# Uncomment to send python tracebacks to the browser if an error occurs:
-# import cgitb; cgitb.enable()
-
 from mercurial import demandimport
 
 demandimport.enable()

tests/test-clone-cgi.t

@@ -12,8 +12,6 b' initialize repository'
   $ cat >hgweb.cgi <<HGWEB
   > #
   > # An example CGI script to use hgweb, edit as necessary
-  > import cgitb
-  > cgitb.enable()
   > from mercurial import demandimport; demandimport.enable()
   > from mercurial.hgweb import hgweb
   > from mercurial.hgweb import wsgicgi

tests/test-mq.t

@@ -1581,8 +1581,6 b' Test that secret mq patch does not break'
   > from mercurial import demandimport; demandimport.enable()
   > from mercurial.hgweb import hgweb
   > from mercurial.hgweb import wsgicgi
-  > import cgitb
-  > cgitb.enable()
   > app = hgweb(b'.', b'test')
   > wsgicgi.launch(app)
   > HGWEB

tests/test-newcgi.t

@@ -9,9 +9,6 b' before d74fc8dec2b4 still work.'
   > #
   > # An example CGI script to use hgweb, edit as necessary
   > 
-  > import cgitb
-  > cgitb.enable()
-  > 
   > from mercurial import demandimport; demandimport.enable()
   > from mercurial.hgweb import hgweb
   > from mercurial.hgweb import wsgicgi
@@ -35,9 +32,6 b' before d74fc8dec2b4 still work.'
   > #
   > # An example CGI script to export multiple hgweb repos, edit as necessary
   > 
-  > import cgitb
-  > cgitb.enable()
-  > 
   > from mercurial import demandimport; demandimport.enable()
   > from mercurial.hgweb import hgwebdir
   > from mercurial.hgweb import wsgicgi

tests/test-newercgi.t

@@ -9,9 +9,6 b' This is a rudimentary test of the CGI fi'
   > #
   > # An example CGI script to use hgweb, edit as necessary
   > 
-  > import cgitb
-  > cgitb.enable()
-  > 
   > from mercurial import demandimport; demandimport.enable()
   > from mercurial.hgweb import hgweb
   > from mercurial.hgweb import wsgicgi
@@ -32,9 +29,6 b' This is a rudimentary test of the CGI fi'
   > #
   > # An example CGI script to export multiple hgweb repos, edit as necessary
   > 
-  > import cgitb
-  > cgitb.enable()
-  > 
   > from mercurial import demandimport; demandimport.enable()
   > from mercurial.hgweb import hgwebdir
   > from mercurial.hgweb import wsgicgi

tests/test-oldcgi.t

@@ -8,8 +8,7 b' This tests if CGI files from before d0db'
   > #
   > # An example CGI script to use hgweb, edit as necessary
   > 
-  > import cgitb, os, sys
-  > cgitb.enable()
+  > import os, sys
   > 
   > # sys.path.insert(0, "/path/to/python/lib") # if not a system-wide install
   > from mercurial import hgweb
@@ -30,8 +29,7 b' This tests if CGI files from before d0db'
   > #
   > # An example CGI script to export multiple hgweb repos, edit as necessary
   > 
-  > import cgitb, sys
-  > cgitb.enable()
+  > import sys
   > 
   > # sys.path.insert(0, "/path/to/python/lib") # if not a system-wide install
   > from mercurial import hgweb

tests/test-push-cgi.t

@@ -16,8 +16,6 b' initialize repository'
 create hgweb invocation script
 
   $ cat >hgweb.cgi <<HGWEB
-  > import cgitb
-  > cgitb.enable()
   > from mercurial import demandimport; demandimport.enable()
   > from mercurial.hgweb import hgweb
   > from mercurial.hgweb import wsgicgi