MIPS: Remove unused PIC macros

asm/asm.h contains CPRESTORE, CPADD & CPLOAD macros that are intended
for use with position independent code, but are not used anywhere in the
kernel - along with a comment to that effect. Remove the dead code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20903/
Cc: linux-mips@linux-mips.org