powerpc: Remove unused st_le*() and ld_le* functions

The powerpc specific st_le*() and ld_le*() functions in
arch/powerpc/asm/swab.h no longer have any users.  They are also
misleadingly named, since they always byteswap, even on a little-endian
host.

This patch removes them.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>