Remove mention of Gitter from contributor docs (#12188)

@ericholscher mentioned that it's not very active, and we can remove the
mention. :)

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:books: Documentation previews :books:

- User's documentation (`docs`):
https://docs--12188.org.readthedocs.build/12188/

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- Developer's documentation (`dev`):
https://dev--12188.org.readthedocs.build/12188/

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Signed-off-by: Pavithra Eswaramoorthy <[email protected]>

Author: pavithraes

docs/dev/contribute.rst

@@ -19,11 +19,9 @@ communication channels:
 
 - Ask usage questions ("How do I?") on `StackOverflow`_.
 - Report bugs, suggest features, or view the source code `on GitHub`_.
-- Discuss development topics on `Gitter`_.
 
 .. _StackOverFlow: https://stackoverflow.com/questions/tagged/read-the-docs
 .. _on GitHub: https://github.com/readthedocs/readthedocs.org
-.. _Gitter: https://gitter.im/readthedocs/community
 
 Contributing
 ------------