switch sum_utils to use AMReX volwgtsum

[zingale]

AMReX now has a volwgtsum function:
AMReX-Codes/amrex#2961

we should be able to use this in place of the Castro version in sum_utils.cpp

[maximumcats]

I'd have to remember more closely why I did #2158 in order to get to #2164, but just worth noting for later that AMReX now does this a little different from Castro. Some uses of volWgtSum in Castro (though perhaps not the ones in sum_utils.cpp) might require an update to the AMReX version to take an Array4 representing the mask.