Reverted "Fixed #25388 -- Added an option to allow disabling of migrations during test database creation"

This reverts commit 157d7f1f since it
disables migrations all the time, not just during tests.

Follow up to 67d98441.

When both parent and child models had managers with the same name and
a migrations opt-in both were added to the migration state.

Thanks Tim for the review.

Thanks Josh for the amazing testing setup and Tim for the review.

It should help catch bugs.

Thanks Tim Graham for the review.

This removes the dependency of the Index class on its model attribute
when a name is passed to it.

Thanks to Markush for discussions.

Thanks to Markush for discussions.