[Form] form_debug.xml should be loaded only if form config is enabled

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |  -
| License       | MIT
| Doc PR        | -

Explication here:
https://github.com/symfony/symfony/commit/1972a916539d0bb877df9b16d373c272edd20cfd#commitcomment-4187567