mtd: partitions: add helper for deleting partition

There are two similar functions handling deletion. One handles single
partition and another the whole MTD flash device. They share (duplicate)
some code so it makes sense to add a small helper for that part.

Function del_mtd_partitions has been moved a bit to keep all deleting
stuff together.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>