mtd: denali: remove unnecessary stubs

This driver uses NAND_ECC_HW_SYNDROME mode.  The nand_scan_tail()
function would not complain about missing ecc->calculate,
ecc->correct, ecc->hwctl handlers.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>