MIPS: Remove unused CAT macro

asm/asm.h provides a CAT macro which is unused throughout the tree, and
if anyone wanted it the generic CONCATENATE macro in linux/kernel.h
provides the same functionality. Delete the dead code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20905/
Cc: linux-mips@linux-mips.org