mem: Re-remove the arch/isa_traits.hh include in the base prefetcher.

mem: Re-remove the arch/isa_traits.hh include in the base prefetcher.

This was removed but then accidentally re-added by a following change,
probably from a slighly incorrect rebase.

Change-Id: Ia7e8c755f92343c8b5e82febea2c1db4683fa69a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34166
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>