Return value of display-scheme-music

[David Kastrup]

It turns out that display-scheme-music, unlike typical display-*
functions, returns the value it displays.  The LilyPond code base
(including regtests) does not make use of that feature, and it is
totally disruptive when used on the Guile command line (because then any
use of display-scheme-music will be followed by the Guile-internal
printed representation of the music object).

I am removing this misfeature (returning the object instead of nothing).
I am writing up an article at the moment, and it would be a total
embarrassment to have to explain the mess that this produces on the
output.

Just in case anybody is surprised that I am not even interested in
wasting the effort of coaching a sanity change like that through the
proper channels.

-- 
David Kastrup