deskutils/mozo: switch to GitHub release asset

Minor versions of 1.28.x are no longer published to the MATE mirror
and are only available on GitHub. Use USE_GITHUB=nodefault with a
release tarball instead of the auto-generated one, as recommended
by the porter's handbook. Remove autoreconf, not needed with release
tarballs. Remove opt-2.pyc entries from pkg-plist, not generated by
the release tarball build.