MIPS: Drop CONFIG_RUNTIME_DEBUG & debug.h

The debug.h header provided some MIPS-specific debug macros, which are
no longer used at all. Remove them.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10693/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>