Remove append_view_path from api ControllerSetup

That line was preventing us from overriding templates within other
application or extensions

Noticed it while trying to override an api rabl template from within another gem/extension. Also I'm still able to override templates from within the rails application removing these lines.

It would be great to have a spec to cover this but I'm not sure how we can do that. Any ideas? In case I didn't miss anything we need to apply it to 2-0-stable, 1-3-stable and 1-2-stable as well. Please let me know if yo want me to submit a PR for each branch