let KeyErrors pass silently on callback removal

I've run into a spurious KeyError when I remove an event connection during another event.  This patch fixes that by letting it pass silently.  I don't have a failing test written as the contribution guidelines request; I'm hoping this commit can be accepted despite that as it is pretty innocent & writing such a test seems to be breaking very new ground -- there is only one test containing "mpl_connect" and simulating events with-out picking a backend is unknown to me.