Remove set namespace to read-only state

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/468497. This MR removes the read-only state of the namespace when Block Seats Overages is active and the namespace is over the seats limit.

MR acceptance checklist

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