fix support level for HTTPRequestHeaderFilter

As noted in kubernetes-sigs#655, a core filter with extended support fields is
confusing and also seems to be incorrect.
This patch drops the extended field from the filter and is a partial fix
for the referenced issue.

HTTPRequestMirroFilter has not been patch as GEP-718 could potentially
simplify that.

apis/v1alpha2/httproute_types.go

@@ -606,8 +606,6 @@ type HTTPRequestHeaderFilter struct {
 	//   GET /foo HTTP/1.1
 	//   my-header: bar
 	//
-	// Support: Extended
-	//
 	// +optional
 	Set []HTTPHeader `json:"set,omitempty"`
 
@@ -627,8 +625,6 @@ type HTTPRequestHeaderFilter struct {
 	//   my-header: foo
 	//   my-header: bar
 	//
-	// Support: Extended
-	//
 	// +optional
 	Add []HTTPHeader `json:"add,omitempty"`
 
@@ -650,8 +646,6 @@ type HTTPRequestHeaderFilter struct {
 	//   GET /foo HTTP/1.1
 	//   my-header2: bar
 	//
-	// Support: Extended
-	//
 	// +optional
 	// +kubebuilder:validation:MaxItems=16
 	Remove []string `json:"remove,omitempty"`