CHUI-601 : Fixed : Crash when dismissing a conversation while a participant is selected

DeletePointer() for the items was replaced by Function LLFolderViewItem::destroyView():
Function LLFolderViewItem::destroyView(), before removing the item, immediately removes it from the Selection List of the Root Folder. As a result, at the time of the call of sanitizeSelection(), will not need to try to handle the elements that have already been deleted.