Remove obsolete mpd scripts.

These scripts should no longer be used as their maintenance has moved
to the "mpdhacks" repository.  We can add that as a git submodule to
achieve a similar level of convenience.

Fix up all the rootmenu scripts to follow the new world order by adding
a new global script that removes most of this code duplication.

As the mpdmenu.pl script is the only user of the MUSIC environment
variable, and it has its own fallback, let's remove the toplevel
assignment of this variable too.