Migrate all users of opt_vec to owned_slice, delete opt_vec.

syntax::opt_vec is now entirely unused, and so can go.

src/libsyntax/lib.rs

@@ -50,7 +50,6 @@ pub mod syntax {
 }
 
 pub mod owned_slice;
-pub mod opt_vec;
 pub mod attr;
 pub mod diagnostic;
 pub mod codemap;