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.
Could there ever be a treeinfo file with more than one file reference that ends in this? I feel like we've seen that before, but maybe it was when "kickstart" repos used to be nested under the "os" repos?