FIX: bare functions in callback registry on bogus unregister

7de483b5fdaa99fa289b418ce0efc5102287ba64 added checking for the case
where the callback was wrapped by backcall, however if there are
un-wrapped functions registered and a non-registered function is
attempted to be unregistered then this fails by raise an uncaught
AttributeError.

IPython/core/events.py

@@ -69,8 +69,11 b' class EventManager(object):'
 
         # Remove callback in case ``function`` was adapted by `backcall`.
         for callback in self.callbacks[event]:
+            try:
             if callback.__wrapped__ is function:
                 return self.callbacks[event].remove(callback)
+            except AttributeError:
+                pass
 
         raise ValueError('Function {!r} is not registered as a {} callback'.format(function, event))