Removes `cached_issues_state_count` feature flag

What does this MR do and why?

Related to #333089 (closed)

Changes from !67418 (merged), that cache the group issues state counts, have been in production since Sep 2, 2021 4:08am GMT+0100 behind the FF cached_issues_state_count.

This FF was enabled in production scoped to group gitlab-org on Sep 6, 2021 and enabled globally on Oct 4, 2021.

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