Remove mention of Wagtail Google group

This list has now been retired.

docs/support.md

@@ -8,10 +8,6 @@ If you have any problems or questions about working with Wagtail, you are invite
 
 [Stack Overflow](https://stackoverflow.com/questions/tagged/wagtail) is the best place to find answers to your questions about working with Wagtail - there is an active community of Wagtail users and developers responding to questions there. When posting questions, please read Stack Overflow's advice on [how to ask questions](https://stackoverflow.com/help/how-to-ask) and remember to tag your question with "wagtail".
 
-## Mailing list
-
-For topics and discussions that do not fit Stack Overflow's question-and-answer format, there is a Wagtail Support mailing list at [groups.google.com/d/forum/wagtail](https://groups.google.com/g/wagtail).
-
 (slack)=
 
 ## Slack