Remove request store cache from virtual registries setting model

What does this MR do and why?

This MR is a follow up from this comment.

The usage of the request store to cache the query is unnecessary, ActiveRecord caching works the same way here.

• Remove cached for group method
• Updates places where this is used, to use find_for_group directly.
• Updates stubs in spec to use find for group.

References

Screenshots or screen recordings

Show in validation steps

How to set up and validate locally

The virtual registries setting toggle should continue work.

1. Go to http://gdk.test:3000/groups/<top level group>/-/settings/packages_and_registries eg http://gdk.test:3000/groups/twitter/-/settings/packages_and_registries
2. Toggle switch and check network response is successful

MR acceptance checklist

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Related to #554069 (closed)