Remove empty matches

[DColadas]

When a websocket is closed, it is removed from the match it was in. However, empty matches do not get removed from MatchManager, and so continue to exist even if they are empty. While there is still not a way to define custom settings for a match, not deleting matches does not only take memory but would keep custom settings defined.

A way to solve this is by leaving a match through MatchManager, instead of by calling MatchHandler's leave(). Whenever a match is removed, a log message should be output, too.