Alternative to #929 to solve issues with array Quantity objects

Rather than transform Quantity to a Numpy array sub-class, we actually *remove* ``__array__``, which solves the issue first reported in #899:

    In [2]: (3. * u.m) * np.array([1,2,3])
    Out[2]: <Quantity [ 3.  6.  9.] m>

    In [3]: np.array([1,2,3]) * (3. * u.m)
    Out[3]: <Quantity [ 3.  6.  9.] m>

This is actually the safest option, i.e. **never** implicitly convert to a Numpy array dropping the units.

(I'm preparing an email to astropy-dev about this)