[1.7.x] Fixed #23938 -- Added migration support for m2m to concrete fields and vice versa

Thanks to Michael D. Hoyle for the report and Tim Graham for the review.

Backport of 623ccdd598625591d1a12fc1564cf3ef9a87581f from master
Revert "[1.7.x] Fixed #23938 -- Added migration support for m2m to concrete fields and vice versa"

This reverts commit 1702bc52cc20ed0729893177fc8f4391b4b3183c.

This doesn't work on stable/1.7.x because #23844 wasn't backported and we're
not willing to do so because it's a large change.