Feature request: Allow accounts to be deactivated when the user is removed from an external auth provider

This issue has been migrated from [#5285](https://github.com/matrix-org/synapse/issues/5285).

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**Description:**

Currently, I'm using Synapse together with [mxisd](kamax-matrix/mxisd) as identity server (which uses LDAP as backend). It would be great if Synapse could occasionally check if the users still exist according to the identiy server, so that accounts can be automatically deactivated. A similar idea to https://docs.nextcloud.com/server/15/admin_manual/configuration_user/user_auth_ldap_cleanup.html, but then for identity servers in general instead of explicitly LDAP.