all: cleanup unused code, part 1 (#10661)

Run `staticcheck` with `U1000` to find unused code. This cleans up about
a half of it. I'll do the other half separately to keep PRs manageable.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

tailcfg/tailcfg_test.go

@@ -4,7 +4,6 @@
 package tailcfg_test
 
 import (
-	"encoding"
 	"encoding/json"
 	"net/netip"
 	"os"
@@ -639,30 +638,6 @@ func TestNetInfoFields(t *testing.T) {
 	}
 }
 
-type keyIn interface {
-	String() string
-	MarshalText() ([]byte, error)
-}
-
-func testKey(t *testing.T, prefix string, in keyIn, out encoding.TextUnmarshaler) {
-	got, err := in.MarshalText()
-	if err != nil {
-		t.Fatal(err)
-	}
-	if err := out.UnmarshalText(got); err != nil {
-		t.Fatal(err)
-	}
-	if s := in.String(); string(got) != s {
-		t.Errorf("MarshalText = %q != String %q", got, s)
-	}
-	if !strings.HasPrefix(string(got), prefix) {
-		t.Errorf("%q didn't start with prefix %q", got, prefix)
-	}
-	if reflect.ValueOf(out).Elem().Interface() != in {
-		t.Errorf("mismatch after unmarshal")
-	}
-}
-
 func TestCloneUser(t *testing.T) {
 	tests := []struct {
 		name string