MINOR: quic: simplify qc_send_hdshk_pkts() return

MINOR: quic: simplify qc_send_hdshk_pkts() return

Clean up trailer of qc_send_hdshk_pkts() by removing label "leave". Only
"out" label is now used. This operation is safe as LIST_DEL_INIT() is
idempotent. Caller of qc_send_hdshk_pkts() also ensures input frame
lists are freed, so it's better to always reset quic_enc_level
<retrans_frms> member.

Also take the opportunity to reset QUIC_FL_CONN_RETRANS_OLD_DATA only if
already set. This is considered more robust and will also remove
unneeded trace occurences.

No functional change. The main objective of this commit is to clean up
code in preparation of a refactoring on send functions.