Remove "show_relevant_approval_rule_approvers" feature flag

What does this MR do and why?

Contributes to #329153 (closed)

It removes feature flag "show_relevant_approval_rule_approvers" that was enabled by default in !72837 (merged)

Screenshots or screen recordings

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How to set up and validate locally

https://docs.gitlab.com/ee/user/project/merge_requests/approvals/rules.html#add-an-approval-rule

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