Remove `group_name_path_vue` feature flag

What does this MR do and why?

Related to #363623 (closed)

In !88085 (merged) and !90295 (merged) we refactored the Group name and Group URL fields that are shown when creating a group, importing a group, or creating/editing a group in the Admin area from HAML/jQuery to Vue. The refactor has now been enabled for all of %15.2 so we would like to remove the feature flag.

Unfortunately there are still a couple of places that still use the jQuery so we can't yet remove that but I have created #367564 as a follow-up.

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