auto: Remove unused pthread spinlock checks

[ac000]

    When configuring under Linux we always got the following
    
      checking for pthread spinlock zero initial value ... found but is not working
    
    Having *actually* taken a look at this, this check seems somewhat bogus,
    the first thing it does is
    
      pthread_spinlock_t  lock = 0;
    
    which you shouldn't do anyway, you should use pthread_spin_init(3) to
    initialise the pthread_spinlock_t variable.
    
    But in any case, this thing, NXT_HAVE_PTHREAD_SPINLOCK_ZERO, isn't even
    checked for in the code.
    
    Neither is NXT_HAVE_PTHREAD_SPINLOCK, we don't use the pthread_spin_*
    API, but rather roll our own spinlock implementation.
    
    So let's just remove these checks, at the very least it'll speed
    ./configure up!