[BC Break] Remove annotations autoloading

Before this PR, Admingenerator automatically load for you annotation routes style. But this supposed a dependency to FrameworkExtraBundle.

IMO, this is not a good idea. Wy should we care about "annotations" and not... REST routes for example?

So here, I removed this "automatic" loader. It's now up to the user to import its own annotations.

This introduce a BC Break, but I'm not sure this is a very used case. IMO, developper should be in situation to manage its own routes.

How to fix BC Break? Simply add resources you want to import annotation routes in your routing.yml file (for required directories). Example:

```yaml
AcmeDemoBundle_admin_acme_demo_bundle_Product:
    resource: "@AcmeDemoBundle/Controller/Product/"
    type:     admingenerator
    prefix:   /admin/acme_demo_bundle/Product

AcmeDemoBundle_admin_acme_demo_bundle_Product_annotation:
    resource: "@AcmeDemoBundle/Controller/Product/"
    type:     annotation
```

PR linked to:
 - issue #698
 - PR #702


Waiting for feedbacks before merging it