Revert sdsc's custom amber pkg to support expanse/0.17.3/cpu/b

After successfully using one of the latest amber Spack packages
developed by Jerry Greenberg here at SDSC to deploy a GPU-accelerated
version of Amber22 and AmberTool23 into expanse/0.17.3/gpu/b, it was
found this package was incapable of redeploying the same CPU-only
version into expanse/0.17.3/cpu/b. As such, we've had to revert to the
original configuration of the package used to deploy into
expanse/0.17.3/cpu/b. We'll eventaully need to reconcile these issues as
neither of these packages support older versions of Amber either. See
[1] for more information.

[1] #50

Author: mkandes