_context.py: Remove useless `return` from `Context.message`

This pointless bare `return` was causing modern pylint to raise an
error.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

buildstream/_context.py

@@ -364,7 +364,6 @@ class Context():
         assert self._message_handler
 
         self._message_handler(message, context=self)
-        return
 
     # silence()
     #