Fix removing of pushers when using workers

[erikjohnston]

Fixes:

2017-06-07 13:42:02,380 - twisted - 131 - CRITICAL -  - Unhandled error in Deferred:
2017-06-07 13:42:02,380 - twisted - 131 - CRITICAL -  - 
Traceback (most recent call last):
  File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1299, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/python/failure.py", line 393, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "synapse/util/metrics.py", line 58, in measured_func
  File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1301, in _inlineCallbacks
    result = g.send(result)
  File "synapse/replication/tcp/resource.py", line 231, in on_remove_pusher
AttributeError: 'ReplicationStreamer' object has no attribute 'notifier'

[NegativeMjark]

LGTM