Unload cray-libsci for whitebox testing.

This was previously done when testing the cray compiler (cce) on
whiteboxes. However, cray-libsci's precense started having issues with other
programming environments recently. So, it is time to unload it for all envs.

It is worth noting, internally the Chapel binary release for Cray systems is
built and tested with cray-libsci module loaded.