MIPS: Remove unused MOVN & MOVZ macros

We have macros in asm/asm.h to allow for use of the MOVN & MOVZ
instructions with compare-and-branch sequences providing compatibility
for ISA versions which don't include those instructions. However the
macros are unused, and appear to have always been unused. Delete the
dead code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20909/
Cc: linux-mips@linux-mips.org