powerpc: Remove CONFIG_POWER3

Now that we have dropped power3 support we can remove CONFIG_POWER3. The
usage in pgtable_32.c was already dead code as CONFIG_POWER3 was not
selectable on PPC32.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>