gui2/mp_alerts_options: Use labelless checkboxes

This fixes an issue where the checkboxes are slightly misaligned
relative to the headings because of the extra space they reserve to the
right.

data/gui/window/mp_alerts_options.cfg

@@ -27,7 +27,7 @@
 
 			[toggle_button]
 				id = {ID} + "_sound"
-				definition = "default"
+				definition = "no_label"
 			[/toggle_button]
 		[/column]
 
@@ -40,7 +40,7 @@
 
 			[toggle_button]
 				id = {ID} + "_notif"
-				definition = "default"
+				definition = "no_label"
 			[/toggle_button]
 		[/column]
 
@@ -53,7 +53,7 @@
 
 			[toggle_button]
 				id = {ID} + "_lobby"
-				definition = "default"
+				definition = "no_label"
 			[/toggle_button]
 		[/column]
 	[/row]