Remove all gogoproto extensions

This commit removes the following gogoproto extensions;

- gogoproto.nullable
- gogoproto.customename
- gogoproto.unmarshaller_all
- gogoproto.stringer_all
- gogoproto.sizer_all
- gogoproto.marshaler_all
- gogoproto.goproto_unregonized_all
- gogoproto.goproto_stringer_all
- gogoproto.goproto_getters_all

None of them are supported by Google's toolchain (see #6564).

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

api/services/namespaces/v1/namespace.proto

@@ -60,7 +60,7 @@ message GetNamespaceRequest {
 }
 
 message GetNamespaceResponse {
-	Namespace namespace = 1 [(gogoproto.nullable) = false];
+	Namespace namespace = 1;
 }
 
 message ListNamespacesRequest {
@@ -68,15 +68,15 @@ message ListNamespacesRequest {
 }
 
 message ListNamespacesResponse {
-	repeated Namespace namespaces = 1 [(gogoproto.nullable) = false];
+	repeated Namespace namespaces = 1;
 }
 
 message CreateNamespaceRequest {
-	Namespace namespace = 1 [(gogoproto.nullable) = false];
+	Namespace namespace = 1;
 }
 
 message CreateNamespaceResponse {
-	Namespace namespace = 1 [(gogoproto.nullable) = false];
+	Namespace namespace = 1;
 }
 
 // UpdateNamespaceRequest updates the metadata for a namespace.
@@ -88,7 +88,7 @@ message UpdateNamespaceRequest {
 	// Namespace provides the target value, as declared by the mask, for the update.
 	//
 	// The namespace field must be set.
-	Namespace namespace = 1 [(gogoproto.nullable) = false];
+	Namespace namespace = 1;
 
 	// UpdateMask specifies which fields to perform the update on. If empty,
 	// the operation applies to all fields.
@@ -100,7 +100,7 @@ message UpdateNamespaceRequest {
 }
 
 message UpdateNamespaceResponse {
-	Namespace namespace = 1 [(gogoproto.nullable) = false];
+	Namespace namespace = 1;
 }
 
 message DeleteNamespaceRequest {