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Clarify/simplify slot_cul source terms

Two things change that do not affect the solution:

(1) The momentum source already had density multiplying the values
coming from function ic() evaluating the values of conserved variables
and were erroneously multiplied by density again, however, the density
is 1.0 for this case, so this makes no difference, and

(2) The total energy source evaluates to zero so the code is removed.