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.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

app/views/groups/_import_group_from_file_panel.html.haml

@@ -14,5 +14,12 @@
         - link_end = '</a>'.html_safe
         = s_('GroupsNew|This feature is deprecated and replaced by %{docs_link_start}group migration%{docs_link_end}.').html_safe % { docs_link_start: docs_link_start, docs_link_end: link_end }
     = render 'shared/groups/group_name_and_path_fields', f: f
+    .form-group
+      = f.label :file, s_('GroupsNew|Upload file')
+      .gl-font-weight-normal
+        - import_export_link_start = '<a href="%{url}" target="_blank">'.html_safe % { url: help_page_path('user/group/settings/import_export') }
+        = s_('GroupsNew|To import a group, navigate to the group settings for the GitLab source instance, %{link_start}generate an export file%{link_end}, and upload it here.').html_safe % { link_start: import_export_link_start, link_end: '</a>'.html_safe }
+      .gl-mt-3
+        = render 'shared/file_picker_button', f: f, field: :file, help_text: nil, classes: 'gl-button btn-confirm-secondary gl-mr-2'
   .gl-border-gray-100.gl-border-solid.gl-border-1.gl-bg-gray-10.gl-p-5
     = f.submit _('Import'), class: 'btn gl-button btn-confirm'