util/mak: delete long-deprecated, unused, pre-generics NonNil func

Updates #5590 (which deprecated it, 2.5 years ago)

Change-Id: I137e82855ee33d91e5639b909f7ca64e237ed6ba
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/mak/mak_test.go

@@ -40,35 +40,6 @@ func TestSet(t *testing.T) {
 	})
 }
 
-func TestNonNil(t *testing.T) {
-	var s []string
-	NonNil(&s)
-	if len(s) != 0 {
-		t.Errorf("slice len = %d; want 0", len(s))
-	}
-	if s == nil {
-		t.Error("slice still nil")
-	}
-
-	s = append(s, "foo")
-	NonNil(&s)
-	if len(s) != 1 {
-		t.Errorf("len = %d; want 1", len(s))
-	}
-	if s[0] != "foo" {
-		t.Errorf("value = %q; want foo", s)
-	}
-
-	var m map[string]string
-	NonNil(&m)
-	if len(m) != 0 {
-		t.Errorf("map len = %d; want 0", len(s))
-	}
-	if m == nil {
-		t.Error("map still nil")
-	}
-}
-
 func TestNonNilMapForJSON(t *testing.T) {
 	type M map[string]int
 	var m M