Remove first_regular_rule method
Related issue: #33329 (closed)
This method has been added for backward compatibility before introduction of approval rules: https://gitlab.com/gitlab-org/gitlab/blob/e095f4ee1e45353a5435a7a5030d6f6000b2d868/ee/app/models/approval_state.rb#L174
It's used in API: https://gitlab.com/gitlab-org/gitlab/blob/e095f4ee1e45353a5435a7a5030d6f6000b2d868/ee/lib/ee/api/entities/approval_state.rb#L33; however the fields are not documented: https://docs.gitlab.com/ee/api/merge_request_approvals.html#merge-request-level-mr-approvals. And they are always empty because first regular rule is any-approver-rule which has no users and groups.
Let's remove it in 16.0.