mtd: nand: mxc: remove now unused code

Since we now have our own read_page/write_page functions
mxc_nand_command() will no longer be called with NAND_CMD_READ0,
NAND_CMD_READOOB, NAND_CMD_SEQIN and NAND_CMD_PAGEPROG. Remove
the code handling these commands.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>