Remove duplicated arc_wake.rs

The auxiliary file arc_wake.rs is in run-pass/auxiliary and also
ui/async-await/auxiliary. Remove the former one as their contents
are same.
Move run-pass/futures-api.rs to ui/async-await/futures-api.rs as it
needs to use arc_wake.rs.

Author: agnxy

src/test/run-pass/auxiliary/arc_wake.rs

@@ -1,3 +1,5 @@
+// run-pass
+
 // aux-build:arc_wake.rs
 
 extern crate arc_wake;