Protected branches can be deleted by setting an MR to remove the source branch
If you create an MR from a protected branch with the 'remove source branch' property checked somehow, this will remove the source branch.
Previously, we checked if a branch was protected before removing it in the backend, but this was removed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11000.
We should add that check back, and require an explicit param (set only on the protected branches page) to be passed in order to skip that check.