remove unused channel_monitor.py

src/wormhole/channel_monitor.py

@@ -1,16 +0,0 @@
-from __future__ import print_function, unicode_literals
-import sys
-from weakref import ref
-
-class ChannelMonitor:
-    def __init__(self):
-        self._open_channels = set()
-    def add(self, w):
-        wr = ref(w, self._lost)
-        self._open_channels.add(wr)
-    def _lost(self, wr):
-        print("Error: a Wormhole instance was not closed", file=sys.stderr)
-    def close(self, w):
-        self._open_channels.discard(ref(w))
-
-monitor = ChannelMonitor() # singleton