powerpc/40x: Remove support for ISS Simulator

ISS4xx has support for 405GP which is obsolete.

Remote it.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7380974bf5952af825ae2552d0a987c0c1c8b506.1590079969.git.christophe.leroy@csgroup.eu