dropping vmpk in favour of jack-keyboard

[Ricardo Wurmus]

Hi Guix,

a while ago I noticed that the software provided by the “vmpk” package
doesn’t work.  It is supposed to create a window displaying a MIDI
keyboard that can be used to send MIDI messages to other applications
via JACK, but it does nothing at all.  I tried to debug this but I
cannot seem to find the cause.

Now that we have “jack-keyboard”, which works just fine, I’m thinking
about just removing “vmpk”.  What do you think?  Any objections?

~~ Ricardo