derp: remove protocol version 1 support

It hasn't existed for a long time and there are no current users.

Fixes #199

derp/derp.go

@@ -44,10 +44,6 @@ const (
 //   * version 2: received packets have src addrs in frameRecvPacket at beginning
 const protocolVersion = 2
 
-const (
-	protocolSrcAddrs = 2 // protocol version at which client expects src addresses
-)
-
 // frameType is the one byte frame type at the beginning of the frame
 // header.  The second field is a big-endian uint32 describing the
 // length of the remaining frame (not including the initial 5 bytes).

derp/derp_server.go

@@ -615,9 +615,7 @@ func (c *sclient) handleFrameSendPacket(ft frameType, fl uint32) error {
 
 	p := pkt{
 		bs:  contents,
-	}
+		src: c.key,
-	if dst.info.Version >= protocolSrcAddrs {
-		p.src = c.key
 	}
 	return c.sendPkt(dst, p)
 }