src: remove unused uv.h include in async-wrap.h

I cannot find any usage of uv in the header and think that it can be
removed.

PR-URL: https://github.com/nodejs/node/pull/12973
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/async-wrap.h

@@ -25,7 +25,6 @@
 #if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
 
 #include "base-object.h"
-#include "uv.h"
 #include "v8.h"
 
 #include <stdint.h>