It was used by ENABLE_DEBUG_TIMING and marked UNUSED if not enabled.

It was actually used in an error message.  I removed that.  I've
never seen that particular message.

This is a separate commit because I can't test it
and I'm not confident it is correct.

It may actually require more than one if you are using refclocks.
I don't understand that area very well.  I think it uses a
receive buffer to hold a partial packet.

I think it was leftover from before kernels provided
timestamps on received packets.

It was useful before kernels timestamped network packets.

The problem persists in read_refclock_packet because the
kernel doesn't timestamp serial data.  There is no easy
way to get the correct/best timestamp.