Cleanup: Removed unused and misnamed or miscalculated variable.

* Removed all references to unused variable totalCO2MolesConsumed from
  PlantImpl.java, as it is either misnamed or miscalculated.  (It's
  moles per square meter as calculated, and without waterFraction.)

* Left totalCO2GramsConsumed, which is correctly calculated from
  myMolesOfCO2Inhaled.

* Parallel to O2: We have totalO2GramsProduced but no
  totalO2MolesProduced.