MIPS: Get rid of CONFIG_CPU_HAS_LLSC

CONFIG_CPU_HAS_LLSC duplicated the function of cpu_has_llsc for no good
reason and and the results if the one was enabled and the other disabled
was very unobvious.  Remove it now that there are no more remaining users.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>