powerpc/32s: Remove CONFIG_PPC_BOOK3S_6xx

As 601 is gone, CONFIG_PPC_BOO3S_6xx and CONFIG_PPC_BOOK3S_32
are dedundant.

Remove CONFIG_PPC_BOOK3S_6xx.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f18c16af37f6f77b577bed8d9e12831b695617ae.1603348103.git.christophe.leroy@csgroup.eu