Fix checkout command if the default branch is not "master"

lab/mergerequestcheckout.py

@@ -79,7 +79,6 @@ class MergeRequestCheckout(RepositoryConnection):
                 print("Aborting")
                 sys.exit(1)
 
-            self._local_repo.refs.master.checkout()
             self._local_repo.delete_head(self.__mr.source_branch, "-f")
 
         head = self._local_repo.create_head(self.__mr.source_branch, fetch_info.ref)