Removed test for em/tests/test_densities.py

It tested for an underflow of np.log. However, this is not true on 64
bit, and we should not test this sort of things.

From: Fabian Pedregosa <fabian.pedregosa@inria.fr>

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@586 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8