pmb.helpers.other: drop old migration code

Get rid of 3 years and older migration code, so we don't need to maintain it anymore. If the user should have run "pmbootstrap init" 3 years ago and suddenly upgrades to pmbootstrap v3, then they will just need to delete their work dir (as pmbootstrap instructs to do) and start over. This is fine, the work dir is just a bunch of caches and locally built packages.