Drop undocumented lilymidi and lilysong

Drop undocumented lilymidi and lilysong

Both have been introduced by commit d0c106f039 ("Singing support")
and haven't received many updates since then. They are mentioned
nowhere in the documentation and there are no regression tests to
make sure they work correctly. Also remove elisp/lilypond-song.el
which called the lilysong script.
This still leaves ly/festival.ly, not documented either. At least
there are some regression tests including it, so it may be working
as intended.

[pkx166h-lilypond, 2019-12-14 15:32:10]

See: https://sourceforge.net/p/testlilyissues/issues/1245/

This is the state of festival.

[lemzwerg, 2019-12-15 06:37:39]

LGTM, thanks.