cleanup: move bogon checking code in netxlite (#562)

I develop this diff while working on https://github.com/ooni/probe/issues/1803#issuecomment-957323297.

While there, make sure we don't have duplicate bogon code
and always use the code inside netxlite.

internal/netxlite/bogon_test.go

@@ -0,0 +1,18 @@
+package netxlite
+
+import "testing"
+
+func TestIsBogon(t *testing.T) {
+	if IsBogon("antani") != true {
+		t.Fatal("unexpected result")
+	}
+	if IsBogon("127.0.0.1") != true {
+		t.Fatal("unexpected result")
+	}
+	if IsBogon("1.1.1.1") != false {
+		t.Fatal("unexpected result")
+	}
+	if IsBogon("10.0.1.1") != true {
+		t.Fatal("unexpected result")
+	}
+}