Remove redundant PreserveLoggingContext

Both (!) things that register as replication listeners do the right thing wrt
logcontexts, so this is redundant.

synapse/notifier.py

@@ -203,7 +203,9 @@ class Notifier(object):
 
     def add_replication_callback(self, cb):
         """Add a callback that will be called when some new data is available.
-        Callback is not given any arguments.
+        Callback is not given any arguments. It should *not* return a Deferred - if
+        it needs to do any asynchronous work, a background thread should be started and
+        wrapped with run_as_background_process.
         """
         self.replication_callbacks.append(cb)
 
@@ -515,12 +517,5 @@ class Notifier(object):
 
     def notify_replication(self):
         """Notify the any replication listeners that there's a new event"""
-        with PreserveLoggingContext():
-            # the callbacks may well outlast the current request, so we run
-            # them in the sentinel logcontext.
-            #
-            # (ideally it would be up to the callbacks to know if they were
-            # starting off background processes and drop the logcontext
-            # accordingly, but that requires more changes)
         for cb in self.replication_callbacks:
             cb()