misc: Use a Makefile to ensure util/packet_pb2.py is up to date.

Rather than just ensuring that packet_pb2.py is available in general, use a
Makefile to ensure that it's also up to date in case packet.proto has
changed.

Also, remove a check that ensures that the protobuf module is available,
since python will complain if it needs it and can't find it.

Finally, remove a comment which talks about manually regenerating the
packet_pb2.py module, something that hasn't been necessary for a while, even
with the old version of this code.

Change-Id: I40c5c1f577e6d7ad1af9a209309a1eb92f073317
Reviewed-on: https://gem5-review.googlesource.com/5005
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>