[ios] Fix toolbar hiding logic in Settings

This patch fixes a  bug in the logic of SettingsRootTableViewController
causing the toolbar to be displayed for settings-related controllers
that do not need a toolbar.

The bug affected settings controllers that were removed from the
the navigation controller while still in editing mode. UIKit was
invoking setEditing:NO while the settings controller was being removed
from the navigation controller stack. This was showing the toolbar when
the removed controller required one and then the next controller in the
stack was left with a toolbar.

The fix makes sure setEditing is not invoked once the controller is
about to be removed from the navigation controller stack, by disabling
editing mode when the controller is about to be removed from the stack.

Fixed: 1404111
Change-Id: If7f916fd15e157d2b7e6b2c9dd2d444ef191ac88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4133396
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Ernesto Izquierdo Clua <eic@google.com>
Cr-Commit-Position: refs/heads/main@{#1089262}