tree-wide: remove (void) cast around sync()

The call is void anyway, it doesn't return an failure indication. Hence,
no need to cast void to (void)...

(We got this right in most cases, but forgot some)

src/basic/async.c

@@ -72,7 +72,7 @@ int asynchronous_sync(pid_t *ret_pid) {
                 return r;
         if (r == 0) {
                 /* Child process */
-                (void) sync();
+                sync();
                 _exit(EXIT_SUCCESS);
         }