Remove feature flags for runner token prefix

What does this MR do and why?

Remove feature flags for runner token prefix, as per this roll out issue: https://gitlab.com/gitlab-org/gitlab/-/issues/353805#remove-feature-flag-definition

All MR's related to this work:

• master !81839 (merged) 👈 - this MR
• 14-8-stable-ee !82119 (merged)
• 14-7-stable-ee !82121 (merged)
• 14-6-stable-ee !82122 (merged)

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