memblock: Reimplement memblock_enforce_memory_limit() using __memblock_remove()

With recent updates, the basic memblock operations are robust enough
that there's no reason for memblock_enfore_memory_limit() to directly
manipulate memblock region arrays.  Reimplement it using
__memblock_remove().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>