Drop down list: stop using has_minimum="false" and remove a workaround

Has_minimum="false" caused the following bug:

1. In a dropdown with toggle buttons, select any entry.
2. Close the dropdown and open it again.
3. Deselecting the entry selected in 1 is now impossible.

This commit fixes the bug above and addresses a FIXME.

data/gui/window/drop_down_list.cfg

@@ -72,7 +72,6 @@ min(
 					[listbox]
 						id = "list"
 						definition = "default"
-						has_minimum = "false"
 
 						[list_definition]
 

src/gui/dialogs/drop_down_menu.cpp

@@ -61,18 +61,6 @@ namespace {
 			}
 		}
 
-		/* FIXME: This dialog uses a listbox with 'has_minimum = false'. This allows a listbox to have 0 or more selections,
-		 * and selecting the same entry toggles that entry's state (ie, if it was selected, it will be deselected). Because
-		 * of this, selecting the same entry in the dropdown list essentially sets the list's selected row to -1, causing problems.
-		 *
-		 * In order to work around this, we first manually deselect the selected entry here. This handler is called *before*
-		 * the listbox's click handler, and as such the selected item will remain toggled on when the click handler fires.
-		 */
-		const int sel = list.get_selected_row();
-		if(sel >= 0) {
-			list.select_row(sel, false);
-		}
-
 		SDL_Rect rect = window.get_rectangle();
 		if(!sdl::point_in_rect(coordinate, rect)) {
 			window.set_retval(window::CANCEL);