MIPS: Drop Loongson _CACHE_* definitions

_CACHE_CACHABLE_NONCOHERENT is defined as 3<<_CACHE_SHIFT by default, so
there's no need to define it as such specifically for Loongson.

_CACHE_CACHABLE_COHERENT is not used anywhere in the kernel, so there's
no need to define it at all.

Finally the comment found alongside these definitions seems incorrect -
it suggests that we're defining _CACHE_CACHABLE_NONCOHERENT such that it
actually provides coherence, but the opposite seems to be true & instead
the unused _CACHE_CACHABLE_COHERENT is defined as the typically
incoherent value.

Delete the whole thing, which will have no effect on the compiled code
anyway.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: linux-mips@vger.kernel.org