formatting: drop `grey`, our custom black version

Now that the official black has all we want, we can drop this.

black.toml

@@ -11,7 +11,6 b' build/'
 | mercurial/thirdparty/
 | hgext/fsmonitor/pywatchman/
 | contrib/python-zstandard/
-| contrib/grey.py
 '''
 skip-string-normalization = true
 quiet = true

tests/test-check-code.t

@@ -21,7 +21,6 b' New errors are not allowed. Warnings are'
   Skipping contrib/automation/hgautomation/try_server.py it has no-che?k-code (glob)
   Skipping contrib/automation/hgautomation/windows.py it has no-che?k-code (glob)
   Skipping contrib/automation/hgautomation/winrm.py it has no-che?k-code (glob)
-  Skipping contrib/grey.py it has no-che?k-code (glob)
   Skipping contrib/packaging/hgpackaging/downloads.py it has no-che?k-code (glob)
   Skipping contrib/packaging/hgpackaging/inno.py it has no-che?k-code (glob)
   Skipping contrib/packaging/hgpackaging/py2exe.py it has no-che?k-code (glob)

tests/test-check-format.t

@@ -1,5 +1,5 b''
 #require black
 
   $ cd $RUNTESTDIR/..
-  $ black --config=black.toml --check --diff `hg files 'set:**.py - hgext/fsmonitor/pywatchman/** - mercurial/thirdparty/** - "contrib/python-zstandard/**" - contrib/grey.py'`
+  $ black --config=black.toml --check --diff `hg files 'set:**.py - hgext/fsmonitor/pywatchman/** - mercurial/thirdparty/** - "contrib/python-zstandard/**"'`
 

tests/test-check-module-imports.t

@@ -20,7 +20,6 b' outputs, which should be fixed later.'
   > -X setup.py \
   > -X contrib/automation/ \
   > -X contrib/debugshell.py \
-  > -X contrib/grey.py \
   > -X contrib/hgweb.fcgi \
   > -X contrib/packaging/hg-docker \
   > -X contrib/packaging/hgpackaging/ \

tests/test-check-py3-compat.t

@@ -6,7 +6,6 b''
 #if no-py3
   $ testrepohg files 'set:(**.py)' \
   > -X contrib/automation/ \
-  > -X contrib/grey.py \
   > -X contrib/packaging/hgpackaging/ \
   > -X contrib/packaging/inno/ \
   > -X contrib/packaging/wix/ \