MINOR: quic: extract qc_stream_buf free in a dedicated function

MINOR: quic: extract qc_stream_buf free in a dedicated function

On ACK reception, data are removed from buffer via qc_stream_desc_ack().
The buffer can be freed if no more data are left. A new slot is also
accounted in buffer connection pool. Extract this operation in a
dedicated private function qc_stream_buf_free().

This change should have no functional change. However it will be useful
for the next patch which needs to remove a buffer from another function.

This patch is necessary for the following bugfix. As such, it must be
backported with it up to 2.6.