Remove exact_matches_first_group_transfer feature flag

What does this MR do and why?

This feature flag appears to be working well on GitLab.com, so we can remove it. There have 2,884 hits over the last 24 hours, all successful, with no noticeable changes in response times.

Relates to:

• #424242 (closed)
• #423958 (closed)

MR acceptance checklist

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