src: remove unused comment

Now that Context::New(Isolate*) returns Local, the removed comment is
no longer applicable that there is a Persistent which will need to be
disposed.
net: readd DTRACE_NET_SOCKET_(READ|WRITE) probes
dtrace: unify dtrace and systemtap interfaces
systemtap: add tapset for node user probes

This makes systemtap tracing prettier, you can do things like `stap -e
'probe node_http_server_request { println(probestr); }'`