Revert "Suggest using Arc on !Send/!Sync types"

This reverts commit 9de1a47.

Author: dtolnay

core/src/marker.rs

@@ -76,11 +76,8 @@ macro marker_impls {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Send")]
 #[rustc_on_unimplemented(
-    on(_Self = "std::rc::Rc<T, A>", note = "use `std::sync::Arc` instead of `std::rc::Rc`"),
     message = "`{Self}` cannot be sent between threads safely",
-    label = "`{Self}` cannot be sent between threads safely",
-    note = "consider using `std::sync::Arc<{Self}>`; for more information visit \
-            <https://doc.rust-lang.org/book/ch16-03-shared-state.html>"
+    label = "`{Self}` cannot be sent between threads safely"
 )]
 pub unsafe auto trait Send {
     // empty.
@@ -631,11 +628,8 @@ impl<T: ?Sized> Copy for &T {}
         any(_Self = "core::cell::RefCell<T>", _Self = "std::cell::RefCell<T>"),
         note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead",
     ),
-    on(_Self = "std::rc::Rc<T, A>", note = "use `std::sync::Arc` instead of `std::rc::Rc`"),
     message = "`{Self}` cannot be shared between threads safely",
-    label = "`{Self}` cannot be shared between threads safely",
-    note = "consider using `std::sync::Arc<{Self}>`; for more information visit \
-            <https://doc.rust-lang.org/book/ch16-03-shared-state.html>"
+    label = "`{Self}` cannot be shared between threads safely"
 )]
 pub unsafe auto trait Sync {
     // FIXME(estebank): once support to add notes in `rustc_on_unimplemented`