It is important to destroy data related to repo mirrors when they are
disabled.
Use `_destroy` nested attribute instead of `enabled` for push mirrors.
Call `remove_import_data` after saving a project if its pull mirror is
disabled.
The `only_protected_branches` value was being enabled by default
even though the checkbox was default off. In EE, this wasn't a
problem because there was EE-specific code to update the
entire form.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53515
