mtd: rawnand: Remove unused NAND_CONTROLLER_ALLOC flag

Looks like NAND_CONTROLLER_ALLOC has been introduced a long time ago
back when the dummy nand_hw_ctrl object was dynamically allocated
instead of being embedded in nand_chip.

We can safely get rid of this unused flag.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>