Resolve flaky test

What does this MR do and why?

ActiveRecord descendants tracker in Ruby 3.1 uses Ruby's native way of tracking subclasses. This bypasses our test, leaving temporary tests models in the hierarchy.

We can't use DescendantsTracker.clear because it is disabled when cache classes is enabled.

The workaround is to leave the temp model in the tree, but remove its dependency on the feature flags.

Reproduce:

With Ruby 3.1 execute:

bundle exec rspec spec/models/concerns/ci/partitionable/switch_spec.rb spec/lib/generators/gitlab/partitioning/foreign_keys_generator_spec.rb

On master it will fail, but on this branch it will pass.

See #429877 (closed)

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Related to #429877 (closed)