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);
         }
 

src/systemctl/systemctl-util.c

@@ -921,7 +921,7 @@ int halt_now(enum action a) {
         /* The kernel will automatically flush ATA disks and suchlike on reboot(), but the file systems need
          * to be synced explicitly in advance. */
         if (!arg_no_sync && !arg_dry_run)
-                (void) sync();
+                sync();
 
         /* Make sure C-A-D is handled by the kernel from this point on... */
         if (!arg_dry_run)