Add missing extra "greenthread" arg to remove_event()

See: http://eventlet.net/doc/modules/greenthread.html
the callback gets passed a "gt" arg.

Change-Id: I9bd44857662e45a1da2d4287017eb966dea8276d
Closes-bug: #1379072
(cherry picked from commit c73d607246b6bec3f5c79fdcee57adf5cfa3cc19)