arch: Eliminate an unused pair of constants from isa_traits.hh.

arch: Eliminate an unused pair of constants from isa_traits.hh.

The one questionable use of CurThreadInfoImplemented (always false) and
CurThreadInfoReg (always -1) has been eliminated, making these constants
unnecessary.

Change-Id: Ibfe4f7be7ce5aaf9c5e896146e1b05b3ac752305
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32922
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>