mem: Merge the virtual Request constructors.

mem: Merge the virtual Request constructors.

The only difference was whether the the atomic op functor was accepted
as an argument. If it wasn't, setVirt would be called without an op
functor argument where it will default to nullptr.

This change deletes the constructor which doesn't take an atomic op
functor and in the other defaults the functor to nullptr. Functionally
nothing changes, but the code is now simpler.

Change-Id: Iff06543b1046594df297344e16961ee9d0f0a373
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26231
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>