1099600 - fix treeinfo files during upgrades

If a user has a 2.3 repo with a treeinfo file that contains repomd.xml checksum
definitions, the upgrade to 2.4 will fail during publish.

To fix this, we need to fix on-disk treeinfo files to remove the offending
checksum definition, and then fix any distribution units to remove the defined
file.

Note that this fix needs to happen before the "new yum distributor" migration,
which is why the migration script ordering was juggled a bit.
I browsed around in a few yum repos to find an an example of multiple repomd.xml files being defined but didn't see any. I think it should be ok to not sync them though as long as pulp is generating its own.