Streamline dashboardMenus cache management

Removed the function `ConcreteInterfaceHelper::cacheInterfaceItems`, since the caching of the `dashboardMenus` session variable is being built by the `ConcreteDashboardHelper::getDashboardAndSearchMenus` function.

Also, `ConcreteInterfaceHelper::clearInterfaceItemsCache` has been simplified, and it's now the only place where the `dashboardMenus` session variable is unset.