[1.7.x] Revert "Fixed #19303 -- Fixed ModelAdmin.formfield_overrides on fields with choices"

This reverts commit 9d1987d7679165ad3a7c2b713a8a488cc1421905.

Backport of 5046c110cf from master
[1.7.x] Removed an unused import

Backport of a31d7c48b0 from master
[1.7.x] Fixed #22690 -- Added a check for proxy models containing fields.

Removed the FieldError raised by ModelBase.__new__ in this case.

Backport of ce993efda8 from master
[1.7.x] Fixed #22748 -- Corrected post_migrate usage example.

Thanks Rudolph for the report.

Backport of a00efa30d6 from master
[1.7.x] Fixed #22753 -- Documented ability to use a note with the {% comment %} tag.

Backport of e020894470 from master