MIPS: Remove code protected by CONFIG_SYS_HAS_CPU_RM9000.

RM9000 support was removed a while ago but this bit crept back in through
commit 69f24d17

 [MIPS: Optimize current_cpu_type() for better code.] which
had been developed before but merged after RM9000 support was removed.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Reported-by: Paul Bolle <pebolle@tiscali.nl>