Cannot detect soundness issue when (get-model) is in the mutant.

[muchang]

When there is a (get-mode) in the seeds, the mutant will also bring a (get-model) after mutation.
If we use this mutant to test SMT solvers, the solvers would report:

unsat
(error "line 13 column 10: model is not available")

or

sat
(error "Cannot get model when produce-models options is off.")

Since they contain error, such outputs will be ignored by yinyang even though this mutant triggers a soundness bug.
We could get rid of (get-model) in our mutants.