MINOR: quic: remove unneeded notification of txbuf room

MINOR: quic: remove unneeded notification of txbuf room

When a stream buffer is freed, qc_stream_desc notify MUX. This is useful
if MUX is waiting for Tx buffer allocation.

Remove this notification in qc_stream_desc(). This is because the
function is called when all stream data have been acknowledged and thus
notified. This function can also be called with some data
unacknowledged, but in this case this is only true just before
connection closure. As such, it is useful to notify the MUX in this
condition.