#11652 [HttpKernel] Remove unused method Kernel::isClassInActiveBundle

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes, deprecation targets 3.0
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #11652
| License       | MIT
| Doc PR        | - (Not mentioned in the docs)


we don't deprecate fixtures. It is not needed as this is not part of the public codebase but of the testsuite