doc: make gendoc.py module import policy aware

Without this, running gendoc.py during an install without C modules
available (via `make local`) will result in an import failure because
the default module load policy insists on C modules.

We also remove the sys.path adjustment because it is no longer needed
since our magic importer handles things.

doc/gendoc.py

@@ -4,10 +4,12 b' where DOC is the name of a document'
 """
 
 import os, sys, textwrap
+
+# This script is executed during installs and may not have C extensions
+# available. Relax C module requirements.
+os.environ['HGMODULEPOLICY'] = 'allow'
 # import from the live mercurial repo
 sys.path.insert(0, "..")
-# fall back to pure modules if required C extensions are not available
-sys.path.append(os.path.join('..', 'mercurial', 'pure'))
 from mercurial import demandimport; demandimport.enable()
 from mercurial import minirst
 from mercurial.commands import table, globalopts