MIPS: Remove unused PREF, PREFE & PREFX macros

asm/asm.h provides PREF(), PREFE() & PREFX() macros which are now
entirely unused. Delete the dead code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20908/
Cc: linux-mips@linux-mips.org