net/{dnscache,interfaces}: use netaddr.IP.IsPrivate, delete copied code

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dnscache/dnscache_test.go

@@ -14,24 +14,6 @@ import (
 
 var dialTest = flag.String("dial-test", "", "if non-empty, addr:port to test dial")
 
-func TestIsPrivateIP(t *testing.T) {
-	tests := []struct {
-		ip   string
-		want bool
-	}{
-		{"10.1.2.3", true},
-		{"172.16.1.100", true},
-		{"192.168.1.1", true},
-		{"1.2.3.4", false},
-	}
-
-	for _, test := range tests {
-		if got := isPrivateIP(net.ParseIP(test.ip)); got != test.want {
-			t.Errorf("isPrivateIP(%q)=%v, want %v", test.ip, got, test.want)
-		}
-	}
-}
-
 func TestDialer(t *testing.T) {
 	if *dialTest == "" {
 		t.Skip("skipping; --dial-test is blank")