No delete button for a content reference
To delete a value from your "Content Reference" field, simply leave the textfield of the desired row-to-delete blank (the multi-field widget for autocomplete fields doesn't have a delete button, it infers which rows should be maintained/deleted based on the presence of a value).

You've marked that field as required (notice the red asterisk at the top), so it must contain at least one value for the form to pass validation. If that's undesirable, just visit the manage fields page, edit the field, and remove the required constraint.