To delete bookmarks from the library:

Select the bookmark or bookmarks you want to delete.
You can select bookmarks in the table or icon view.

Use the menu entry Edit > Delete or press the Delete key.
If you change your mind, choose Edit > Undo.
If you no longer use a bookmark you can easily remove it from the library. You also can only remove a bookmark from a collection.
Select the bookmark or bookmarks you want to delete.
You can select bookmarks in the table or icon view.
Use the menu entry Edit > Delete or press the Delete key.
If you change your mind, choose Edit > Undo.
By pressing and holding the Option Key at the same time you can remove a bookmark from the collection and the library.
You can not remove bookmarks from smart collections or folder.