Deprecate IOSource::IsPacketSource()?

[timwoj]

From #4567 (comment):

Total fly-by skimming: Could we deprecate IsPacketSource() below? Seems unused and also seems a bit strange that a consumer working with IOSources should know whether it's a packet source and even our own packet sources do not override. So, maybe just ditch it right away? It hasn't worked for years.

Somewhere in the IO loop there's iosource == pkt_src and I suspect IsPacketSource() might be reminiscent of that.