When all plugins have failed, MPD used to fall back to the "mad"
decoder plugin, to handle those radio streams without a Content-Type
response header.  This however leads to unexpected results (garbage
being played) when the stream isn't really mp3.  Since we care little
about "bad" streams, we shouldn't have hacks which have bad side
effects.

Let's get rid of this hack now!  Only try to "mad" plugin if there was
no match at all (Content-Type, path suffix) and no other plugin has
been tried.