[3.0][HttpKernel] Remove unused method Kernel::isClassInActiveBundle

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes, if using isClassInActiveBundle (not used by Symfony itself)
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #11652
| License       | MIT
| Doc PR        | - (Not mentioned in the docs)

Follow-up of #11869