old source tarballs

[Graham Percival]

We need to make more space on lilypond.org.  I've deleted
binaries, documentation, and regtests from everything other than
the first and last release of a stable series.

I'm going to do the same for the source tarballs, but I'll wait a
few days in case anybody urgently wants to save them.  This should
be unnecessary because all that info should be in git, but better
safe than sorry.

- Graham