[cleanup] remove solidsystem from modeltraits

The model traits somehow had the number of solidcomponents which was not used anywhere. Also I am very confused why this worked to begin with. The equilibrium model traits don't have a solidsystem, or am I missing something? When coupling the mineralization model with the nonequilibrium model an error occurred but I don't get why it does not for all models. Anyway I think it is not needed here and possibly leads to bugs.