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/types/sandbox.proto

@@ -37,16 +37,16 @@ message Sandbox {
 		google.protobuf.Any options = 2;
 	}
 	// Runtime specifies which runtime to use for executing this container.
-	Runtime runtime = 2 [(gogoproto.nullable) = false];
+	Runtime runtime = 2;
 	// Spec is sandbox configuration (kin of OCI runtime spec), spec's data will be written to a config.json file in the
 	// bundle directory (similary to OCI spec).
 	google.protobuf.Any spec = 3;
 	// Labels provides an area to include arbitrary data on containers.
-	map<string, string> labels  = 4 [(gogoproto.nullable) = false];
+	map<string, string> labels  = 4;
 	// CreatedAt is the time the container was first created.
 	google.protobuf.Timestamp created_at = 5;
 	// UpdatedAt is the last time the container was mutated.
 	google.protobuf.Timestamp updated_at = 6;
 	// Extensions allow clients to provide optional blobs that can be handled by runtime.
-	map<string, google.protobuf.Any> extensions = 7 [(gogoproto.nullable) = false];
+	map<string, google.protobuf.Any> extensions = 7;
 }