Remove activerecord-deprecated_finders

The ActiveRecord Deprecated Finders gem can be removed without any other problems. It will have support dropped with the release of Rails 5, and we've already migrated to the new syntax for everything the gem was built to cover.

Results of git grep:

• find_all_by: no results
• find_last_by: no results
• scoped_by: no results
• find_or_initialize_by: All updated to new syntax
• find_or_create_by: All updated to new syntax

It should be perfectly safe to remove. :)