middle-end: removed unused variable in vectorizable_live_operation_1

It looks like the previous patch had an unused variable.
It's odd that my bootstrap didn't catch it (I'm assuming
-Werror is still on for O3 bootstraps) but this fixes it.

gcc/ChangeLog:

	* tree-vect-loop.cc (vectorizable_live_operation_1): Drop unused
	restart_loop.
	(vectorizable_live_operation): Likewise.