Follow-up from "Refactor spec/policies/project_policy_spec.rb to minimize the diff with CE"

The following discussion from !2953 (merged) should be addressed:

• @godfat started a discussion: (+1 comment)

  I am a bit confused about push_code_to_protected_branches. It seems that CE does have some reference to it, but I can't tell if it has any effect.

  Probably not indeed, it seems it's never passed to users_select_data_attributes, in CE nor in EE. Thus, I think push_code_to_protected_branches: options.pushCodeToProtectedBranches || null, in UsersSelect is always set to null. That means we can probably remove prevent :push_code_to_protected_branches from app/policies/project_policy.rb.