tpl: Get rid of the custom template truth logic

Fixes #6615

tpl/internal/go_templates/texttemplate/exec.go

@@ -307,7 +307,7 @@ func IsTrue(val interface{}) (truth, ok bool) {
 	return isTrue(reflect.ValueOf(val))
 }
 
-func isTrue(val reflect.Value) (truth, ok bool) {
+func isTrueOld(val reflect.Value) (truth, ok bool) {
 	if !val.IsValid() {
 		// Something like var x interface{}, never set. It's a form of nil.
 		return false, true