sparc32: Un-btfixup {pte,pmd,pgd}_clear().

Also we can remove BTFIXUPCALL_SWAPO0G0 as that is no longer
used.

This was rather amusing, we were setting the btfixup vectors
based upon cpu type but all to the same exact generic srmmu
routines.

Furthermore, we were inconsistently marking the fixup as
either BTFIXUPCALL_SWAPO0G0 or BTFIXUPCALL_NORM.

What a mess, glad we could untangle this stuff.

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