[SecurityBundle] Firewall providers building - code cleaning
minor #12400 [SecurityBundle] Firewall providers building - code cleaning (blanchonvincent)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #12400).

Discussion
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[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.

Commits
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c3c904d [SecurityBundle] Firewall providers building - code cleaning