MIPS: disable CONFIG_IDE in rbtx49xx_defconfig

rbtx49xx_defconfig enables CONFIG_IDE for the tx4938 and tx4939 ide
drivers, but those aren't actually used by the last known remaining user:

https://lore.kernel.org/lkml/20210107.101729.1936921832901251107.anemo@mba.ocn.ne.jp/Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/configs/rbtx49xx_defconfig

@@ -44,9 +44,6 @@ CONFIG_MTD_NAND_TXX9NDFMC=m
 CONFIG_BLK_DEV_LOOP=y
 CONFIG_BLK_DEV_RAM=y
 CONFIG_BLK_DEV_RAM_SIZE=8192
-CONFIG_IDE=y
-CONFIG_BLK_DEV_IDE_TX4938=y
-CONFIG_BLK_DEV_IDE_TX4939=y
 CONFIG_NETDEVICES=y
 CONFIG_NE2000=y
 CONFIG_SMC91X=y