repo: Stop creating "transaction" symlink
This originally was a way that we detected the case where a pull was
interrupted. Later, we added `.commitpartial` files which also cover
this case.
See also https://github.com/GNOME/ostree/pull/85
We still want to honor their existence (and unlink them) in case an
old version of ostree was in use, but I believe it's safe to stop
creating them now.
The only case where this would break is if you have a version of
ostree that predates commitpartial in your rollback history, but such
old versions are no longer in use by operating systems I support at
least.
Closes: https://github.com/GNOME/ostree/pull/100