MIPS: Malta: remove unused cpu_khz variable

This variable was introduced by commit 96348c8f (of Ralf's historic
Linux/MIPS repository) "Remaining fixes for MIPS's eval boards." but
I don't see any use of it either then or now. Remove it.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6171/