mx_flock: Don't unlock before close

We don't need to call flock(fd, LOCK_UN) for a file we are going to
close anyway, so don't do it.

This might make a difference if the lock fd was dupped into another fd.

mx_flock.c

@@ -149,10 +149,6 @@ int mx_funlock(struct mx_flock *lock)
     if (res < 0)
         mx_log_warning("unlink(): %m");
 
-    res = flock(lock->fd, LOCK_UN);
-    if (res < 0)
-        mx_log_warning("flock(): %m");
-
     _flock_close(lock);
     _flock_free(lock);