Remove usage of enable_add_on_users_pagesize_selection feature flag

What does this MR do and why?

Removes enable_add_on_users_pagesize_selection Feature flag and it's usages.

Changelog: added

EE: true

How to set up and validate locally

1. Visit seat utilisation section of a group with a paid subscription and a gitlab duo product http://localhost:3000/groups//-/settings/gitlab_duo/seat_utilization

2. Ensure the page selection element shows up correctly by default when the list has more than 20 user items

   

MR acceptance checklist

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Related to #504292