refactor(error): resolve unused trait error

Author: tottoto

src/error.rs

@@ -601,11 +601,6 @@ impl From<http::uri::InvalidUriParts> for Parse {
     }
 }
 
-#[doc(hidden)]
-trait AssertSendSync: Send + Sync + 'static {}
-#[doc(hidden)]
-impl AssertSendSync for Error {}
-
 // ===== impl TimedOut ====
 
 impl fmt::Display for TimedOut {
@@ -621,6 +616,13 @@ mod tests {
     use super::*;
     use std::mem;
 
+    fn assert_send_sync<T: Send + Sync + 'static>() {}
+
+    #[test]
+    fn error_satisfies_send_sync() {
+        assert_send_sync::<Error>()
+    }
+
     #[test]
     fn error_size_of() {
         assert_eq!(mem::size_of::<Error>(), mem::size_of::<usize>());