The Delete icon (trash bin) is used to delete an asset(s) in a folder (see screenshot A). When you have selected a collection, the trash bin icon changes to an "X" mark (see screenshot B). To remove an asset from a collection, click the X icon in the top tool bar.
A. If a Subfolder is selected, click the Trashbin icon to delete an asset(s).
B. If a Collection is selected, click the X icon to remove an asset(s) in a Collection.