powerpc: Remove outdated Documentation/powerpc/smp.txt

Documentation/powerpc/smp.txt is so outdated that it makes sense to just
remove it.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>