MIPS: Remove cpu_has_6k_cache and cpu_has_8k_cache in cpu_cache_init()

Since commit 02cf2119 ("Cleanup the mess in cpu_cache_init."),
cpu_has_6k_cache and cpu_has_8k_cache have no user, r6k_cache_init()
and r8k_cache_init() are not defined for over 15 years, just remove
them.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>