mtd: pxa3xx_nand: discard wait_for_event,write_cmd,__readid function

Since we have rework the irq process, we don't need additional
delay in wait_for_event. Also write_cmd and __readid is also
discarded for the same reason.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>