Driver core / MM: Drop offline_memory_block()

Since offline_memory_block(mem) is functionally equivalent to
device_offline(&mem->dev), make the only caller of the former use
the latter instead and drop offline_memory_block() entirely.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Toshi Kani <toshi.kani@hp.com>