[SecurityBundle] minor simplification in main configuration

As per @stof's suggestion (https://github.com/symfony/symfony/pull/9587#discussion_r7875630).

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a