mtd: brcmnand: drop unused subpage_read() support

AFAIR this driver was never tested with subpage read support, and this
code is currently unused because we don't set the NAND_SUBPAGE_READ
flag. It can be resurrected if someone tests it properly.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Tested-by: Ray Jui <rjui@broadcom.com>