mips: Remove unused or misplaced values from isa_traits.hh.

mips: Remove unused or misplaced values from isa_traits.hh.

Most of these values were unused, except the interrupt levels which were
moved to the interrupt controller, the only place they were used.
Unnecessary includes were also removed.

Change-Id: I783966413d51391663a9217ed672ec1f2b4719b7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33197
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>