net: marvell: Remove reference to CONFIG_MV64X60

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

As it is not a user selectable config item, all references to it
are stale. Remove them.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>