Remove xfail from Quantity test_std_inplace since upstream fix is in

Inplace calculations of `std` used to store the variance. This was a `numpy` bug, raised in https://github.com/numpy/numpy/issues/5240 and solved by https://github.com/numpy/numpy/pull/5242. That implies that for numpy >=1.9.1, the test we already had will no longer fail. This PR ensures we will notice if we do something wrong on our side.