Remove Sync bound from Command (#5871)

Unless I'm mistaken it is unnecessary, Commands are never accessed from two threads simultaneously. It unnecessarily restricts Command structs

crates/bevy_ecs/src/system/commands/mod.rs

@@ -40,7 +40,7 @@ use super::Resource;
 ///     commands.add(AddToCounter(42));
 /// }
 /// ```
-pub trait Command: Send + Sync + 'static {
+pub trait Command: Send + 'static {
     fn write(self, world: &mut World);
 }