editors/pluma-plugins: 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 and yelp-tools, not
needed with release tarballs. Remove opt-2.pyc entries from
pkg-plist, not generated by the release tarball build.