Adds removeHandler utility function

Greetings!

This pull request adds the function

    System.Log.Logger.removeHandler :: Logger -> Logger

It removes the handler that was last added to a logger and can therefore be used to undo a call to `addHandler`. The main practical advantage is that `removeHandler` can be used to completely silence the logging system by removing the default handler from the root logger, which should come in handy for programs that implement something like a `-qq` option.

(By the way, the commit description mentions `removeLastHandler` instead of `removeHandler` because I forgot to update it after a rebase. Sorry about that.)