evolve: remove legacy.py that provided debugconvertobsolete

It's not very useful nowadays because the old obsmarkers format was before Mercurial 2.3 and it's pretty safe to say we're not going to see any repo from that era that people tried to use safe history rewriting and still didn't upgrade it.

Additionally, legacy.py was excluded from packages, so it was only easy to use for evolve developers and contributors.