Remove Loader inheritance from CTMLoader

[takahirox]

CTMLoader is the only loader inheriting Loader but it doesn't look it really needs to inherit. CTMLoader calls scope(this).createMaterial() but it can be replaced with THREE.Loader.prototype.createMaterial() without inheriting Loader as similarly LegacyJSONLoader does.

With this change, no loader will inherit Loader. We may remove Loader if we move initMaterials(), createMaterial() and Handlers to somewhere(LoaderUtils?).

[Mugen87]

It's important to keep the loaders consistent so I think this change is a good one.

[mrdoob]

Thanks!