Revert "debian has iptables-legacy and iptables-nft now"

This reverts commit 7da66ee.

Libnetwork should only use the iptables binary. Iptables v1.8 and above
uses the nftables backend. The translations for all the rules used by
libnetwork is supported by the new iptables binary.

Signed-off-by: Arko Dasgupta <arko.dasgupta@docker.com>

Author: Arko Dasgupta

iptables/iptables.go

@@ -87,16 +87,11 @@ func initFirewalld() {
 }
 
 func detectIptables() {
-	path, err := exec.LookPath("iptables-legacy") // debian has iptables-legacy and iptables-nft now
+	path, err := exec.LookPath("iptables")
 	if err != nil {
-		path, err = exec.LookPath("iptables")
-		if err != nil {
-			return
-		}
+		return
 	}
-
 	iptablesPath = path
-
 	supportsXlock = exec.Command(iptablesPath, "--wait", "-L", "-n").Run() == nil
 	mj, mn, mc, err := GetVersion()
 	if err != nil {