MIPS: Remove SLOW_DOWN_IO

arch/mips appears to have inherited SLOW_DOWN_IO from arch/x86 in
antiquity, but we never define CONF_SLOWDOWN_IO so this is unused code.

Perhaps it was once useful to keep the MIPS header close to the x86
version to ease comparisons or porting changes, but they've diverged
significantly at this point & x86 does this differently now anyway.

Delete the dead code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20343/
Cc: linux-mips@linux-mips.org