Remove unneeded bounds on S in FromRequest(Parts) implementations

This doesn't actually allow using non-Send or non-Sync state types, but
it makes the docs simpler.

axum-macros/tests/debug_handler/fail/extract_self_ref.rs

@@ -1,8 +1,4 @@
-use axum::{
-    async_trait,
-    extract::FromRequest,
-    http::Request,
-};
+use axum::{async_trait, extract::FromRequest, http::Request};
 use axum_macros::debug_handler;
 
 struct A;
@@ -11,11 +7,10 @@ struct A;
 impl<S, B> FromRequest<S, B> for A
 where
     B: Send + 'static,
-    S: Send + Sync,
 {
     type Rejection = ();
 
-    async fn from_request(_req: Request<B>, _state: &S) -> Result<Self, Self::Rejection> {
+    async fn from_request(_req: Request<B>, _: &S) -> Result<Self, Self::Rejection> {
         unimplemented!()
     }
 }