remove unused code in d2 solution

src/days/d2.rs

@@ -13,10 +13,6 @@ fn _parse (data: &str) -> I {
     }).collect()).collect()
 }
 
-fn _solve (data: &I) -> O {
-    Default::default()
-}
-
 fn _silver (data: &I) -> O {
     let mut out = format!("");