mtd: nand: atmel_nand: remove build warning in atmel_nand_remove()

mtd: nand: atmel_nand: remove build warning in atmel_nand_remove()

In the 4.9.y tree, the following build warning keeps showing up:

	drivers/mtd/nand/atmel_nand.c:2337:19: warning: unused variable 'mtd' [-Wunused-variable]

This driver was deleted / restructured in newer kernels so this is a
4.9.y patch only.

Cc: Wenyou Yang <wenyou.yang@atmel.com>
Cc: Josh Wu <rainyfeeling@outlook.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>