mem: Delete the MessageReq and MessageResp memory commands.

mem: Delete the MessageReq and MessageResp memory commands.

Now that Message*Port is gone, there are no users of these two memory
commands. They can now be deleted, simplifying the memory system
slightly.

Change-Id: If157dade4a3fb2610756c2ee81dc0c3fac670a26
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20824
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>