MIPS: dbau1x00: Remove unused flash driver stub

MIPS: dbau1x00: Remove unused flash driver stub

All dbau1x00 boards use the CFI driver so this stub driver is useless
and should not be compiled.

This patch fixes the error:

u-boot-git/board/dbau1x00/flash.c:34: multiple definition of `flash_init'
drivers/mtd/libmtd.o:u-boot-git/drivers/mtd/cfi_flash.c:2084: first defined here
board/dbau1x00/libdbau1x00.o: In function `write_buff':
u-boot-git/board/dbau1x00/flash.c:40: multiple definition of `write_buff'
drivers/mtd/libmtd.o:u-boot-git/drivers/mtd/cfi_flash.c:1265: first defined here

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>