MIPS: Cleanup break and trap codes.

Very ancient out-of-tree KDB versions were using BRK_KDB code but it's
unused in modern kernels since a long time.  Delete it.

The microMIPS encoding only reserves 4 bits for a trap code so it's time
for further weedkilling.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>