Goto the URL opera://bookmarks. You can right-click on a folder and choose "move to trash".
To select a bookmark, you can left-click the check for each bookmark you want, right-click one of the selected bookmarks and choose "move selected to trash". Or, you can right-click in a blank spot and chose "select all". Or, you can left-click a blank spot and then hit ctrl + a to select all. Or, you can left-click in a blank spot, hold and drag to put a selection box around the bookmarks you want, which works really well if you change to list view. Del key also works for "move to trash".