Remove the prefix_ci_build_tokens feature flag

What does this MR do and why?

Remove the prefix_ci_build_tokens feature flag

GitLab began prefixing CI Job Tokens when a feature flag was enabled:

• MR: Add a prefix to CI Build tokens behind a featur... (!140159 - merged)
• Rollout issue: [Feature flag] Rollout of `prefix_ci_build_tokens` (gitlab-com/gl-infra/production#17299 - closed)

With the successful rollout we can always prefix CI Job tokens, and remove the feature flag. This enables us to more easily detect leaked secrets.

Resolves gitlab-com/gl-infra/production#17299 (closed) Resolves #426137 (closed)

Changelog: changed

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Related to #17299