MINOR: mux-quic: simplify empty emitted STREAM FIN handling

MINOR: mux-quic: simplify empty emitted STREAM FIN handling

An empty STREAM frame can be emitted by QUIC MUX to notify about a
delayed FIN. This requires tedious comparison in qmux_ctrl_send() to
differentiate between the first empty frame and a retransmitted one.

Simplify this by unsubscribe from streamdesc layer when the QCS is
locally closed after FIN transmission. This prevents all future
retransmitted frames to be notify to the QCS instance, especially any
retransmitted empty FIN.