Remove read-only state banner when BSO

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/468497. This MR is removing code that is not longer needed based the new requirements: https://gitlab.com/gitlab-org/gitlab/-/issues/432733#note_1879579003. Note that this is only removing the code related to the read-only banner. The actual logic that puts namespaces in read-only state will be handled in a separate MR.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the feature flag: Feature.enable(:block_seat_overages, group)
2. Add more billable members to your group than there are seats in the subscription
3. Run gdk in simulating SaaS mode
4. The namespace should not display read-only banners

Note that because this uses reactive caching, the banners may not load on the initial page load, and you need to make sure that your rails background jobs are working locally.