Technical debt: Simplify some classes parameterization

[jcpitre]

Describe the problem

In #1810 we modified some classes to require 2 class parameters, e.g.

public abstract class GtfsEntityContainer<T extends GtfsEntity, D extends GtfsFileDescriptor> {

I believe that the D extends GtfsFileDescriptor is not necessary.

Proposed solution

Removing the second class parameter would simplify the code.

Alternatives you've considered

No response

Additional context

No response