small masked array performance enhancement

remove unnecessary typecast and seterr operations
master:

     In [1]: md = np.ma.masked_array(np.ones(10000), mask=False)
     In [2]: %timeit md + md
     10000 loops, best of 3: 55 µs per loop

branch:

    In [3]: %timeit md + md
    10000 loops, best of 3: 50.6 µs per loop