Remove bound 3pids separately from locally associated 3pids

It's possible for users to bind a 3pid without adding it to
a local association first, thus using `user_get_threepids` may not
result in all known bound
3pids. Which essentially culminates in us not unbinding all 3pids for
an account upon deactivation.

This commit separates unbinding bound 3pids and deleting local assocaitions into two separate steps.

synapse/handlers/deactivate_account.py

@@ -100,10 +100,10 @@ async def deactivate_account(
         # unbinding
         identity_server_supports_unbinding = True
 
-        # Remove any threepids associated with this account locally and attempt to
-        # unbind them from identity server(s).
-        threepids = await self.store.user_get_threepids(user_id)
-        for threepid in threepids:
+        # Attempt to unbind any known bound threepids to this account from identity
+        # server(s).
+        bound_threepids = await self.store.user_get_bound_threepids(user_id)
+        for threepid in bound_threepids:
             try:
                 result = await self._auth_handler.delete_and_unbind_threepid(
                     user_id, threepid["medium"], threepid["address"], id_server
@@ -115,6 +115,13 @@ async def deactivate_account(
 
             identity_server_supports_unbinding &= result
 
+        # Remove any local threepid associations for this account.
+        local_threepids = await self.store.user_get_threepids(user_id)
+        for threepid in local_threepids:
+            await self._auth_handler.delete_and_unbind_threepid(
+                user_id, threepid["medium"], threepid["address"], id_server
+            )
+
         # delete any devices belonging to the user, which will also
         # delete corresponding access tokens.
         await self._device_handler.delete_all_devices_for_user(user_id)