Remove the assignment for real this time.

cc:
https://github.com/rails/rails/commit/a11e2fcff8cdc9691b977d8330abe63024f8c732#commitcomment-6353614
and 0369dbf
Merge pull request #15139 from zzak/habtm_build_model_name

Remove the assignment for real this time.
Remove unecessary require
Merge pull request #15140 from arthurnn/small_cleanup

Remove unecessary require