types/persist: remove Persist.LegacyFrontendPrivateMachineKey

It was a temporary migration over four years ago. It's no longer
relevant.

Updates #610

Change-Id: I1f00c9485fab13ede6f77603f7d4235222c2a481
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -949,8 +949,6 @@ func TestEditPrefsHasNoKeys(t *testing.T) {
 		Persist: &persist.Persist{
 			PrivateNodeKey:    key.NewNode(),
 			OldPrivateNodeKey: key.NewNode(),
-
-			LegacyFrontendPrivateMachineKey: key.NewMachine(),
 		},
 	}).View(), ipn.NetworkProfile{})
 	if p := b.pm.CurrentPrefs().Persist(); !p.Valid() || p.PrivateNodeKey().IsZero() {
@@ -977,10 +975,6 @@ func TestEditPrefsHasNoKeys(t *testing.T) {
 		t.Errorf("OldPrivateNodeKey = %v; want zero", p.Persist().OldPrivateNodeKey())
 	}
 
-	if !p.Persist().LegacyFrontendPrivateMachineKey().IsZero() {
-		t.Errorf("LegacyFrontendPrivateMachineKey = %v; want zero", p.Persist().LegacyFrontendPrivateMachineKey())
-	}
-
 	if !p.Persist().NetworkLockKey().IsZero() {
 		t.Errorf("NetworkLockKey= %v; want zero", p.Persist().NetworkLockKey())
 	}