[wpiutil] Remove Protobuf.getNested() (#6812)

It was superseded by using FileDescriptor.getDependencies().

wpiutil/src/main/java/edu/wpi/first/util/protobuf/Protobuf.java

@@ -48,15 +48,6 @@ public interface Protobuf<T, MessageType extends ProtoMessage<?>> {
    */
   Descriptor getDescriptor();
 
-  /**
-   * Gets the list of protobuf types referenced by this protobuf.
-   *
-   * @return list of protobuf types
-   */
-  default Protobuf<?, ?>[] getNested() {
-    return new Protobuf<?, ?>[] {};
-  }
-
   /**
    * Creates protobuf message.
    *