Remove the assignment for real this time.

cc:
https://github.com/rails/rails/commit/a11e2fcff8cdc9691b977d8330abe63024f8c732#commitcomment-6353614
and 0369dbf

Conflicts:
	activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
Merge pull request #15287 from zzak/backport_14871

Backport 14871