Remove MergeRequest#target_branches and its usages

What does this MR do?

MergeRequestController#update_branches controller action has been removed in 244f511e

Now we can remove the view as well 🚀 The view was the only one which used @target_branches instance variable and this variable was the only one which used MergeRequest#target_branches

The commit also removed the usage of MergeRequest#source_branches. Since we don't use that method anymore, we can delete it as well