server/middleware: fix session usage

server/szurubooru/middleware/db_session.py

@@ -8,5 +8,4 @@ class DbSession(object):
         request.context.session = self._session_factory()
 
     def process_response(self, request, _response, _resource):
-        # any commits need to happen explicitly in the API layer.
-        request.context.session.close()
+        self._session_factory.remove()