Clicking "force remove source branch" label now toggles the checkbox again

We remove the ID from the hidden tag for `merge_request[force_remove_source_branch]`
in order to fix the checkbox toggling when the associated label is clicked.

The issue was introduced by !7267 and discovered in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7267#note_18028311.

See merge request !7356