cleanup: remove unused code in requireDir()

No changes in functionality.
Pass `app` to fn exported by auto-required script

When a script in `models/` or `boot/` exports a function which is not
a loopback.Model constructor, the bootstrapper immediatelly calls
this exported function wit the current `app` object.

This is providing a dependency injection mechanism for boot scripts,
so that they no longer need to know where to find the `app` object.

Note: the dependency injection is optional. Existing code getting
`app` reference via `require('../app')` will continue to work.
Start CHANGES.md