Remove old way of feature flagging once those experiments wrapped up

[jeffwidman]

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Code improvement description

_jurre created a new cleaner way to create/manage feature flags in https://github.com/dependabot/dependabot-core/pull/5755

Once the experiments using the old way of feature flags wind down, we should remove the old way. See more context in https://github.com/dependabot/dependabot-core/pull/5755#issuecomment-1252657232