MINOR: trace: add a trace_no_cb() dummy callback for when to use no callback

MINOR: trace: add a trace_no_cb() dummy callback for when to use no callback

By default, passing a NULL cb to the trace functions will result in the
source's default one to be used. For some cases we won't want to use any
callback at all, not event the default one. Let's define a trace_no_cb()
function for this, that does absolutely nothing.