When setting new TabBar items, remove previous ones first.

Inactive `viewControllers` of my RDVTabBarController subclass get purged when memory is low. And so as its  TabBarItem objects. The problem when reestablishing again those controllers is that the TabBar items of the purged VCs are still out there on the TabBar, resulting in duplicate tabs. When I checked the setter method of RDVTabBar's item property, it only assumes the same items are being sent over when in fact they're not.