Remove KUBECTL_EXPLAIN_OPENAPIV3 environment variable

Since explain openapiv3 has been moved to beta already, there is no
point to having this environment variable.

This PR removes it.

staging/src/k8s.io/kubectl/pkg/cmd/explain/explain.go

@@ -76,8 +76,7 @@ type ExplainOptions struct {
 	Mapper meta.RESTMapper
 	Schema openapi.Resources
 
-	// Name of the template to use with the openapiv3 template renderer. If
-	// `EnableOpenAPIV3` is disabled, this does nothing
+	// Name of the template to use with the openapiv3 template renderer.
 	OutputFormat string
 
 	// Client capable of fetching openapi documents from the user's cluster

staging/src/k8s.io/kubectl/pkg/cmd/explain/explain_test.go

@@ -185,9 +185,7 @@ func TestExplainOpenAPIV3(t *testing.T) {
 	}
 	cases = append(cases, explainV2Cases...)
 
-	cmdtesting.WithAlphaEnvs([]cmdutil.FeatureGate{cmdutil.ExplainOpenapiV3}, t, func(t *testing.T) {
 	runExplainTestCases(t, cases)
-	})
 }
 
 func runExplainTestCases(t *testing.T, cases []explainTestCase) {

staging/src/k8s.io/kubectl/pkg/cmd/util/helpers.go

@@ -426,7 +426,6 @@ type FeatureGate string
 
 const (
 	ApplySet              FeatureGate = "KUBECTL_APPLYSET"
-	ExplainOpenapiV3      FeatureGate = "KUBECTL_EXPLAIN_OPENAPIV3"
 	CmdPluginAsSubcommand FeatureGate = "KUBECTL_ENABLE_CMD_SHADOW"
 	InteractiveDelete     FeatureGate = "KUBECTL_INTERACTIVE_DELETE"
 )