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>