arch-x86: Delete some unused register related constants.
arch-x86: Delete some unused register related constants.

Change-Id: Id5305a863675061b4afb27c71b329180605381b5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39677
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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