Remove docs for Delete Group Admin API (#13112)

This API no longer exists.

Signed-off-by: Aaron Raimist <aaron@raim.ist>

changelog.d/13112.doc

+Remove documentation for the Delete Group Admin API which no longer exists.
\ No newline at end of file

docs/SUMMARY.md

@@ -55,7 +55,6 @@
     - [Admin API](usage/administration/admin_api/README.md)
       - [Account Validity](admin_api/account_validity.md)
       - [Background Updates](usage/administration/admin_api/background_updates.md)
-      - [Delete Group](admin_api/delete_group.md)
       - [Event Reports](admin_api/event_reports.md)
       - [Media](admin_api/media_admin_api.md)
       - [Purge History](admin_api/purge_history_api.md)

docs/admin_api/delete_group.md

-# Delete a local group
-
-This API lets a server admin delete a local group. Doing so will kick all
-users out of the group so that their clients will correctly handle the group
-being deleted.
-
-To use it, you will need to authenticate by providing an `access_token`
-for a server admin: see [Admin API](../usage/administration/admin_api).
-
-The API is:
-
-```
-POST /_synapse/admin/v1/delete_group/<group_id>
-```