Remove locked? check when unlocking a merge request

Our state machine shall have enough knowledge to not allow states other
than locked from transitioning to opened (e.g. MergeRequest#unlock_mr).

app/models/merge_request.rb

@@ -819,7 +819,7 @@ class MergeRequest < ActiveRecord::Base
       lock_mr
       yield
     ensure
-      unlock_mr if locked?
+      unlock_mr
     end
   end