mpd: Remove old exception handling.

This is a leftover from the old dispatcher code that was missed in the
refactor. The call() will never raise a LookupError for missing commands.

mopidy/mpd/dispatcher.py

@@ -169,8 +169,6 @@ class MpdDispatcher(object):
             if exc.command is None:
                 exc.command = tokens[0]
             raise
-        except LookupError:
-            raise exceptions.MpdUnknownCommand(command=tokens[0])
 
     def _format_response(self, response):
         formatted_response = []