Remove various Sync bounds

With `Transport` becoming non-Clone and having `&mut` self receivers,
the `Sync` requirement no longer makes any sense and we can thus
remove it.

core/src/transport.rs

@@ -157,7 +157,7 @@ pub trait Transport {
     /// Boxes the transport, including custom transport errors.
     fn boxed(self) -> boxed::Boxed<Self::Output>
     where
-        Self: Transport + Sized + Send + Sync + Unpin + 'static,
+        Self: Transport + Sized + Send + Unpin + 'static,
         Self::Dial: Send + 'static,
         Self::ListenerUpgrade: Send + 'static,
         Self::Error: Send + Sync,

core/src/transport/boxed.rs

@@ -31,7 +31,7 @@ use std::{
 /// Creates a new [`Boxed`] transport from the given transport.
 pub fn boxed<T>(transport: T) -> Boxed<T::Output>
 where
-    T: Transport + Send + Sync + Unpin + 'static,
+    T: Transport + Send + Unpin + 'static,
     T::Error: Send + Sync,
     T::Dial: Send + 'static,
     T::ListenerUpgrade: Send + 'static,
@@ -45,7 +45,7 @@ where
 /// and `ListenerUpgrade` futures are `Box`ed and only the `Output`
 /// and `Error` types are captured in type variables.
 pub struct Boxed<O> {
-    inner: Box<dyn Abstract<O> + Send + Sync + Unpin>,
+    inner: Box<dyn Abstract<O> + Send + Unpin>,
 }
 
 type Dial<O> = Pin<Box<dyn Future<Output = io::Result<O>> + Send>>;

core/src/transport/upgrade.rs

@@ -295,7 +295,7 @@ impl<T> Multiplexed<T> {
     /// the [`StreamMuxer`] and custom transport errors.
     pub fn boxed<M>(self) -> super::Boxed<(PeerId, StreamMuxerBox)>
     where
-        T: Transport<Output = (PeerId, M)> + Sized + Send + Sync + Unpin + 'static,
+        T: Transport<Output = (PeerId, M)> + Sized + Send + Unpin + 'static,
         T::Dial: Send + 'static,
         T::ListenerUpgrade: Send + 'static,
         T::Error: Send + Sync,