Drop subset tarball and replace it with "configure --enable-subset"

Drop subset tarball and replace it with "configure --enable-subset"

There's no point creating two separate tarfiles.  It's not clear
anyone was actually using the subset tarball --- and if they are, they
can replace it by the full source distribution and using the
--enable-subset option.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>