war on -Werror, part 8: more informals.

General guide: if the parameter name and its typings
match almost exactly, the name goes away.
Otherwise, it's commented out to allow for some
documentation.

src/ScreenOptions.cpp

@@ -753,7 +753,7 @@ void ScreenOptions::AfterChangeValueOrRow( PlayerNumber pn )
 }
 
 
-void ScreenOptions::MenuBack( const InputEventPlus &input )
+void ScreenOptions::MenuBack( const InputEventPlus & )
 {
 	Cancel( SM_GoToPrevScreen );
 }