MINOR: mux-quic: remove unneeded sent-offset fields

MINOR: mux-quic: remove unneeded sent-offset fields

Both QCS and QCC have their owned sent offset field. These fields store
the newest offset sent to the quic-conn layer. It is similar to QCS/QCC
flow control real offset. This patch removes them and replaces them by
the latter for code clarification.

MINOR: mux-quic: remove unneeded qcc.tx.sent_offsets field

This commit as a similar purpose as previous, except that it removes QCC
<sent_offsets> field, now equivalent to connection flow control real
offset.