erdtlssrtpdec: Properly unref some ghosted static pads
erdtlssrtpdec,erdtlssrtpenc: Make dtls element removal a bit more robust

This fixes a hypothetical race if there's data/events going through the
element that trigger the probe before we sent out an EOS. Now we send a
non-sticky serialized custom event, so both cases should work fine -- if
the probe is called before the event is sent, no side effect will take
place, and if not, then our event will trigger the probe.