MINOR: quic: Remove QUIC TX packet length evaluation function

MINOR: quic: Remove QUIC TX packet length evaluation function

Remove qc_eval_pkt() which has come with the multithreading support. It
was there to evaluate the length of a TX packet before building. We could
build from several thread TX packets without consuming a packet number for nothing (when
the building failed). But as the TX packet building functions are always
executed by the same thread, the one attached to the connection, this does
not make sense to continue to use such a function. Furthermore it is buggy
since we had to recently pad the TX packet under certain circumstances.