clean tests/ui/serde.rs

Cleaning the empty lines for clarity.

tests/ui/serde.rs

@@ -37,7 +37,6 @@ impl<'de> serde::de::Visitor<'de> for B {
     }
 
     fn visit_string<E>(self, _v: String) -> Result<Self::Value, E>
-
         where E: serde::de::Error,
     {
         unimplemented!()

tests/ui/serde.stderr

@@ -2,11 +2,10 @@ error: you should not implement `visit_string` without also implementing `visit_
   --> $DIR/serde.rs:39:5
    |
 39 | /     fn visit_string<E>(self, _v: String) -> Result<Self::Value, E>
-40 | |
-41 | |         where E: serde::de::Error,
-42 | |     {
-43 | |         unimplemented!()
-44 | |     }
+40 | |         where E: serde::de::Error,
+41 | |     {
+42 | |         unimplemented!()
+43 | |     }
    | |_____^
    |
 note: lint level defined here