don't print variables in witness that are not part of the formula

[marijnheule]

CaDiCaL prints a value for all variables between 1 and the max variable index. It would be convenient if the variables that do not occur in the formula are skipped when printing the witness.

[arminbiere]

At one point tools used in the competition required to have values for all variables. So adding this feature would still require to determine whether it should be enabled by default or not.