MIPS: Remove unused HIGHMEM_DEBUG macro

We have a HIGHMEM_DEBUG macro defined in asm/highmem.h with a comment
stating that it should be removed for production, and no users... Kill
it.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14523/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>