Stop building rasops4 on amd64.

Stop building rasops4 on amd64.

It was pulled in for efifb, but it is extremely unlikely an EFI system
supporting only 4-bit color depth (16 colors) exists. Even if it existed
though, on SMALL_KERNEL rasops4_putchar() simply returns EAGAIN so it
would not be possible to install the system.

For the record, we do not build rasops4 on i386 or on any of our other
platforms either.

OK kettenis@, mpi@