refactor RTT measurements to simplify the sentPacketHistory

internal/ackhandler/sent_packet_history.go

@@ -23,13 +23,6 @@ func (h *sentPacketHistory) SentPacket(p *Packet) {
 	h.packetMap[p.PacketNumber] = el
 }
 
-func (h *sentPacketHistory) GetPacket(p protocol.PacketNumber) *Packet {
-	if el, ok := h.packetMap[p]; ok {
-		return &el.Value
-	}
-	return nil
-}
-
 // Iterate iterates through all packets.
 // The callback must not modify the history.
 func (h *sentPacketHistory) Iterate(cb func(*Packet) (cont bool, err error)) error {