Revert "mm: Check if section present during memory block (un)registering"

This reverts commit 7568fb63

 as it's
already in Linus's tree through a different patch.

Reported-by: Tony Luck <tony.luck@intel.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: stable@vger.kernel.org #v3.15
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>