Remove bogus "message" trait from IFuture (#394)

The `IFuture` interface declares a `message` trait that was a leftover from refactoring: it's not implemented by any of the concrete implementations, and is not used. This PR removes it.

traits_futures/i_future.py

@@ -14,7 +14,7 @@
 
 import abc
 
-from traits.api import Any, Bool, Event, Interface, Property, Str, Tuple
+from traits.api import Bool, Interface, Property
 
 from traits_futures.future_states import FutureState
 
@@ -42,12 +42,6 @@ class IFuture(Interface):
     #: it will be consistent with the ``state``.
     done = Property(Bool())
 
-    #: Event trait providing custom messages from the background task.
-    #: Subclasses of ``BaseFuture`` can listen to this trait and interpret
-    #: the messages in whatever way they like. Each message takes the
-    #: form ``(message_type, message_args)``.
-    message = Event(Tuple(Str(), Any()))
-
     @property
     @abc.abstractmethod
     def result(self):