LoongArch: Remove MIPS comment about cycle counter

This comment block was taken originally from the MIPS architecture code,
where indeed there are particular assumptions one can make regarding SMP
and !SMP and cycle counters. On LoongArch, however, the rdtime family of
functions is always available. As Xuerui wrote:

    The rdtime family of instructions is in fact guaranteed to be
    available on LoongArch; LoongArch's subsets all contain them, even
    the 32-bit "Primary" subset intended for university teaching -- they
    provide the rdtimeh.w and rdtimel.w pair of instructions that access
    the same 64-bit counter.

So this commit simply removes the incorrect comment block.

Link: https://lore.kernel.org/lkml/e78940bc-9be2-2fe7-026f-9e64a1416c9f@xen0n.name/
Fixes: b738c106

 ("LoongArch: Add other common headers")
Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>