Remove docs for Delete Group Admin API (#13112)

This API no longer exists.

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

docs/admin_api/delete_group.md

@@ -1,14 +0,0 @@
-# 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>
-```