Remove assumption

See https://github.com/element-hq/synapse/pull/19057#discussion_r2427083757

synapse/server.py

@@ -640,9 +640,6 @@ class HomeServer(metaclass=abc.ABCMeta):
         Some handlers have side effects on instantiation (like registering
         background updates). This function causes them to be fetched, and
         therefore instantiated, to run those side effects.
-
-        We assume these background tasks are only run on a single Synapse instance at a
-        time.
         """
         for i in self.REQUIRED_ON_BACKGROUND_TASK_STARTUP:
             getattr(self, "get_" + i + "_handler")()