[SecurityBundle] Firewall providers building - code cleaning

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | https://github.com/symfony-fr/symfony-docs-fr/pull/609

It seems old code to build firewall provider still exist in the SecurityExtension. The `In-Memory` and `Entity` providers are handled in their own factories.
you still need to check whether ``$provider['chain']`` is set. Here, you are trying to build any provider as a chain one, even when the key is not set