rtpjitterbuffer: remove the concept of "already-lost"

rtpjitterbuffer: remove the concept of "already-lost"

This is a concept that only applies when a buffer arrives in the chain
function, and it has already been scheduled as part of a "multi"-lost
timer.

However, "multi"-lost timers are now a thing of the past, making this
whole concept superflous, and this buffer is now simply counted as "late",
having already been pushed out (albeit as a lost-event).