remove superflous case when queueing undecryptable packets
remove superflous case when queueing undecryptable packets

This was only needed in gQUIC. In IETF QUIC, we determine the keys from
the packet header. The crypto setup will return an ErrKeysDropped if we
already dropped the corresponding keys.

lucas-clemente/quic-go,lucas-clemente/quic-go