sparc32: Un-btfixup update_mmu_cache().

The magic Swift SRMMU code in question has not been enabled for
something on the order of a decade, and it as well as it's comment
is there in the history in case we ever need it again.

Therefore all implementations are NOPs and we can kill this stuff
off.

Signed-off-by: David S. Miller <davem@davemloft.net>