#5641 Drop undocumented lilymidi and lilysong

[Jonas Hahnfeld]

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.

http://codereview.appspot.com/571250043

[Anonymous]

Passes make, make test-baseline and a full make doc,

[David Kastrup]

lilymidi works, its man page is sufficient as its documentation (as opposed to that of lilysong) and I frequently use it for dumping midi files. One could mention it in the usage guide but it's functional as well as in active use.

[Jonas Hahnfeld]

Just not tested automatically in any way, as James' result shows. But ok, then we should keep both or why would we drop lilysong if lacking documentation isn't a reason?

[David Kastrup]

lilysong lacks useful information, lilymidi has what it needs in the man page, it's just not cross-referenced. We should probably announce intention to remove lilysong in the LilyPond user mailing list and slate it for 2.21. Unless keeping it alive for 2.20 is too much work, in which case we might remove it earlier.