Hide scroll group selector for 'simple'

[imnasnainaec]

Part 2 of #2249, which has the following commit comment:

Navigation is now exclusively through the main app toolbar. The editor already synced with the main toolbar via scroll group 0; this removes the redundant in-editor BookChapterControl and ScrollGroupSelector

Keeping this in draft because there is a bug in 'simple' mode without the controls:

• Platform Scripture Editor doesn't stay synced with the top-bar control if they are on different scroll groups. That was already an issue, but is made harder to diagnose if this pr removes the scroll group selector.

The second commit of this pr is one proposed solution: adding a listener to the local storage to see when the scroll group changes.

Another possible approach would be to refactor useWebViewScrollGroupScrRef to (e.g.) have two different versions of scrRef and setScrRefWithScroll, one that tracks with the global scroll group (for simple) and one that doesn't (for power).

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This change is