Clean up import-side workarounds for ancient export corruption issues

[dralley]

We can't import exports created by an old version of Pulp. Therefore: once an export-side issue has been fixed, the import-side workarounds can also be removed.

Arguably they should only exist in backports to begin with.

Example, this commit can potentially be reverted in full:

36afce0#diff-a4ef2c2769054500968814758405c1362874f4f02fce5c497d3d92c631aa85d4R467

#2648

[dralley]

We need to be 100% sure why this was there and that it is no longer necessary.