Fix phpdoc and coding standards

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~


This removes the unused use statements which were not catched by PHP-CS-Fixer because of string occurences. It also fixes some invalid phpdoc (scalar is not recognized as a valid type for instance).

This is complementary to https://github.com/symfony/symfony/pull/13134