Add functionality to remove deactivated users from the monthly_active_users table (#10947)

* add test

* add function to remove user from monthly active table in deactivate code

* add function to remove user from monthly active table

* add changelog entry

* update changelog number

* requested changes

* update docstring on new function

* fix lint error

* Update synapse/storage/databases/main/monthly_active_users.py

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>