CLEANUP: quic: drop the intermediate packet tree pointer in the fast retransmissions

CLEANUP: quic: drop the intermediate packet tree pointer in the fast retransmissions

In qc_prep_fast_retrans() and qc_prep_hdshk_fast_retrans(), the local
<pkts> pointer is assigned once and then only used once, as argument of
eb64_first(). Dereference the packet number space tree directly instead,
which removes five lines without changing the behaviour.

No functional change, no need to backport.