Stop handling specialization in clippy's to_string_trait_impl lint

ToString can no longer be specialized, so no need to account for it in
to_string_trait_impl either.

tests/ui/to_string_trait_impl.rs

@@ -30,46 +30,3 @@ impl Bar {
         String::from("Bar")
     }
 }
-
-mod issue12263 {
-    pub struct MyStringWrapper<'a>(&'a str);
-
-    impl std::fmt::Display for MyStringWrapper<'_> {
-        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-            self.0.fmt(f)
-        }
-    }
-
-    impl ToString for MyStringWrapper<'_> {
-        fn to_string(&self) -> String {
-            self.0.to_string()
-        }
-    }
-
-    pub struct S<T>(T);
-    impl std::fmt::Display for S<String> {
-        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-            todo!()
-        }
-    }
-    // no specialization if the generics differ, so lint
-    impl ToString for S<i32> {
-        fn to_string(&self) -> String {
-            todo!()
-        }
-    }
-
-    pub struct S2<T>(T);
-    impl std::fmt::Display for S2<String> {
-        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-            todo!()
-        }
-    }
-
-    // also specialization if the generics don't differ
-    impl ToString for S2<String> {
-        fn to_string(&self) -> String {
-            todo!()
-        }
-    }
-}

tests/ui/to_string_trait_impl.stderr

@@ -12,17 +12,5 @@ LL | | }
    = note: `-D clippy::to-string-trait-impl` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::to_string_trait_impl)]`
 
-error: direct implementation of `ToString`
-  --> tests/ui/to_string_trait_impl.rs:56:5
-   |
-LL | /     impl ToString for S<i32> {
-LL | |         fn to_string(&self) -> String {
-LL | |             todo!()
-LL | |         }
-LL | |     }
-   | |_____^
-   |
-   = help: prefer implementing `Display` instead
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error