Remove imported unsupported threepids when deactivating a user

[sandhose]

With syn2mas, we chose to import unsupported threepids (mainly MSISDNs) in a table, but forgot to remove them when a user deactivate their account, which we should do.

Plan is to ship this in the next release, then clean up the table in the release after that.
The table is small enough even on matrix.org for that cleanup to happen in a migration:

DELETE FROM user_unsupported_third_party_ids
USING users
WHERE users.deactivated_at IS NOT NULL
  AND users.user_id = user_unsupported_third_party_ids.user_id;

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