remove "encryption -> pcap" cruft

As of e12dbb460 (#1758) we no longer force the usage of pcap when
encryption is enabled. Hence...

- we no longer need to figure out whether the router was started with
  a password when re-attaching it from the proxy. We therefore remove
  the code for that.

- there is no longer any mode in which we pass both `--datapath` and
  `--iface` options to the router. Previously this would only happen
  when encryption was enabled and the only overlay used for
  connections was sleeve. It is not clear what the behaviour is when
  these conditions do not hold, so, to be safe, we prevent the
  specification of both flags.

prog/weaver/main.go

@@ -128,23 +128,15 @@ func main() {
 	config.ProtocolMinVersion = byte(protocolMinVersion)
 
 	overlays := weave.NewOverlaySwitch()
-	if datapathName != "" {
-		// A datapath name implies that "Bridge" and "Overlay"
-		// packet handling use fast datapath, although other
-		// options can override that below.  Even if both
-		// things are overridden, we might need bridging on
-		// the datapath.
+	switch {
+	case datapathName != "" && ifaceName != "":
+		Log.Fatal("At most one of --datapath and --iface must be specified.")
+	case datapathName != "":
 		fastdp, err := weave.NewFastDatapath(datapathName, config.Port)
 		checkFatal(err)
 		networkConfig.Bridge = fastdp.Bridge()
 		overlays.Add("fastdp", fastdp.Overlay())
-	}
-	if ifaceName != "" {
-		// -iface can coexist with -datapath, because
-		// pcap-based packet capture is a bit more efficient
-		// than capture via ODP misses, even when using an
-		// ODP-based bridge.  So when using weave encyption,
-		// it's preferable to use -iface.
+	case ifaceName != "":
 		var err error
 		iface, err = weavenet.EnsureInterface(ifaceName)
 		checkFatal(err)

weave

@@ -1622,7 +1622,6 @@ fetch_router_args() {
     CONTAINER_ARGS=$(docker inspect -f '{{.Args}}' $CONTAINER_NAME)
     IPRANGE=$(echo $CONTAINER_ARGS | grep -o -e '-ipalloc-range [^ ]*') || true
     NO_DNS_OPT=$(echo $CONTAINER_ARGS | grep -o -e '--no-dns') || true
-    WEAVE_PASSWORD=$(docker inspect -f '{{.Config.Env}}' $CONTAINER_NAME | sed -n -e 's/.*WEAVE_PASSWORD=\([^] ]*\).*/\1/p')
 }
 
 attach_router() {