mem-ruby: Remove conditional includes based on THE_ISA in ruby.

mem-ruby: Remove conditional includes based on THE_ISA in ruby.

These were including instruction class definitions from x86 for some
reason. There was no code in those .cc files which actually used
anything from them, as evidenced by the fact that the GCN3_X86 build
still works. No other code in the file was conditionally compiled as of
today.

Change-Id: I3cef8348fb601dd7af67665cf64bbf514c91c3db
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34577
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>