Make users not to be considered members of __world__ group

[acelaya]

This PR ensures we do not display the option to leave the "Public" (AKA world) group, as that wouldn't work anyway.

This has been discussed in slack https://hypothes-is.slack.com/archives/C1M8NH76X/p1731690164285009

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.43%. Comparing base (e80f694) to head (d455bf1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6671 +/- ## ======================================= Coverage 99.43% 99.43% ======================================= Files 271 271 Lines 10210 10211 +1 Branches 2431 2431 ======================================= + Hits 10152 10153 +1 Misses 58 58 ```

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[acelaya]

I opened a discussion in slack to decide on the best approach to solve this https://hypothes-is.slack.com/archives/C2C2U40LW/p1732181595497659

I'm closing this for now and will reopen if a decision is made.

[acelaya]

Superseded by https://github.com/hypothesis/client/pull/6684