Remove set_primary_key, replace with primary_key=

First deprecated, now removed: https://github.com/rails/rails/commit/9add7608f1acaa68b025470e7a38901d7e6161ca#activerecord/lib/active_record/attribute_methods/primary_key.rb
Merge pull request #11952 from lmarcetic/master

Remove set_primary_key, replace with primary_key=