powerpc/embedded6xx: Remove CONFIG_MV64X60

Commit 92c8c16f ("powerpc/embedded6xx: Remove C2K board support")
moved the last selector of CONFIG_MV64X60.

As it is not a user selectable config, it can be removed.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Wolfram Sang <wsa@kernel.org> # for I2C
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/19e57d16692dcd1ca67ba880d7273a57fab416aa.1616085654.git.christophe.leroy@csgroup.eu