Remove the unsafe impl Send for ConstellationControlMsg.

This impl made it possible to put raw pointers in ConstellationControlMsg and
send them across threads without considering the consequences.

This required making SmallVec1<T> Send if T is Send.

components/script_traits/lib.rs

@@ -76,9 +76,6 @@ pub enum ConstellationControlMsg {
     UpdateSubpageId(PipelineId, SubpageId, SubpageId),
 }
 
-unsafe impl Send for ConstellationControlMsg {
-}
-
 /// The mouse button involved in the event.
 #[derive(Clone, Debug)]
 pub enum MouseButton {