Remove group_administration_nav_item feature flag

What does this MR do and why?

And it's related code. Apparently we introduced moving some settings menu items, namely SAML SSO, Usage Quotas and Billing, into a new "Administration" menu for group owners. The change was not well-liked and was put behind a feature flag.

The corresponding feature flag group_administration_nav_item is not enabled per default or otherwise. It isn't documented, it is really old, so let us just get rid of it.

See also:

• #373936
• I have found no other issue which directly relates to this feature flag

Screenshots or screen recordings

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How to set up and validate locally

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