sparc: Kill mmu_{un,}lockarea().

These were used on sun4c during floppy data transfers since on that
chip we had to lock the cpu mappings into the TLB because we cannot
take a TLB miss during the assembler floppy interrupt handler that
does the data transfer.

That is no longer necessary since we've removed sun4c support, thus
this stuff can disappear completely.

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