Don't call MainWindow::loadSettings() on any config page change

The addressed and removed TODO and the removed connection were added in dc2f271d: "Port away from KSettings::Dispatcher". I guess the developer was too busy porting to check whether that old code was still useful and had to be ported rather than simply removed.

There is only one UI-related configuration page: UiPreferences. UiPreferences::apply() already calls loadSettings() for each MainWindow. Consequently, if there is a single main window, MainWindow::loadSettings() is called twice (redundantly) when UiPreferences changes and once (uselessly) when some other config page changes. Removing this connection is a perfect fix: now loadSettings() is called once when UiPreferences changes and not called when some other config page changes.