Use gitlab_git for removing a branch instead of gitlab-shell

This was quite low hanging fruit, the method delete_branch is already provided in gitlab_git, so just swap one for the other.

A thing to notice, since this happens in the branches controller, but the deletion happens as a background job, the web page is not updating properly when the branch is deleted. Maybe the question is: is there any way of doing polling from the browser to listen for an event and then cleanup the page?

cc @DouweM