Issue #3589789: Drop orphan '#group' => 'tracking_scope' reference

Closes #3589789.

The linktracking details element referenced a tracking_scope group that is never declared in PianoAnalyticsSettingsForm::buildForm(), in any alter hook in the module, or in any submodule. Drupal's #group resolver silently no-ops a reference whose target does not exist, so the details rendered as a standalone collapsible block. Removing the dead reference does not change the rendered form — linktracking already rendered as a free-standing details — it just keeps the form honest.

If a vertical-tabs layout is wanted later, the tracking_scope parent element can be added back deliberately at that point.