Remove group_iterations feature flag

What does this MR do and why?

Remove the group_iterations feature flag, as it has been enabled for many years

For #221047 (closed)

How to test

Check content loads on iterations pages.

example for groups: http://gdk.test:3000/groups/gitlab-org/-/iterations/
example for projects: http://gdk.test:3000/gitlab-org/gitlab-shell/-/iterations/

Searched codebase for :group_iterations to remove flag, then without the colon for remaining cleanup

MR acceptance checklist

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