[MTD] [NAND] Remove unused variable in plat_nand_remove

With CONFIG_MTD_PARTITIONS not set, got this:
drivers/mtd/nand/plat_nand.c:113: warning: unused variable 'pdata'

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>