Fix zeroing out remote quarantined media count (#19559)

Just something I noticed while working on
https://github.com/element-hq/synapse/pull/19558

We start the function by setting `total_media_quarantined` to zero, then
we do work on the `media_ids`, add the number affected, zero it out
(**bug**), do work on `hashes`, add the number of affected rows, then
return `total_media_quarantined`.

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changelog.d/19559.bugfix

@@ -0,0 +1 @@
+Fix quarantine media admin APIs sometimes returning inaccurate counts for remote media.

synapse/storage/databases/main/room.py

@@ -1217,7 +1217,6 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
             txn.execute(sql, [quarantined_by] + sql_args)
             total_media_quarantined += txn.rowcount if txn.rowcount > 0 else 0
 
-        total_media_quarantined = 0
         if hashes:
             sql_many_clause_sql, sql_many_clause_args = make_in_list_sql_clause(
                 txn.database_engine, "sha256", hashes