CLEANUP: quic: remove the dead encryption level variable in qc_prep_hdshk_fast_retrans()

CLEANUP: quic: remove the dead encryption level variable in qc_prep_hdshk_fast_retrans()

<qel> was assigned once to qc->iel and never modified afterwards, so
every "qel == iqel" test was always true. Use <iqel> directly for the
packet number space and for the pto_probe increment, test the
anti-amplification condition positively, and unwrap the requeue
condition.

The <start> label has had no goto target since this function stopped
looping over encryption levels, remove it.

No functional change, no need to backport.