Revert "MIPS: unhide PATA_PLATFORM"

Revert commit 75b18aac ("MIPS: unhide PATA_PLATFORM") now that
HAVE_PATA_PLATFORM is set selectively for all the relevant platforms.

Verified with `db1xxx_defconfig' and `sb1250_swarm_defconfig' by making
sure PATA_PLATFORM is still there in `.config' with this change applied,
and with `malta_defconfig' by making sure it's now gone.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>