mtd: rawnand: remove unused is_imx51_nfc and imx53_nfc functions

clang with W=1 reports
drivers/mtd/nand/raw/mxc_nand.c:1602:19: error: unused function
  'is_imx51_nfc' [-Werror,-Wunused-function]
static inline int is_imx51_nfc(struct mxc_nand_host *host)
                  ^
drivers/mtd/nand/raw/mxc_nand.c:1607:19: error: unused function
  'is_imx53_nfc' [-Werror,-Wunused-function]
static inline int is_imx53_nfc(struct mxc_nand_host *host)
                  ^
These functions are not used, so remove them.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230321114638.1782086-1-trix@redhat.com