Don't iterate all connectors for unregistering (#14714)

This reduces the time spent in unregisterRemovedConnectors when updating
the text of one Label with about 380 connectors registered from 20 to 2
ms as reported by the Vaadin profiler. Performance when removing lots of
connectors remains at about 10 ms for removing 360 connectors. Profiled
in Firefox 32 on OS X.

Change-Id: I203fd8790f8ccc7c098ee91c44831a5ac6c4a82b