Decide what to do with linear option of stoch_simul

Apparently, we have an undocumented option linear in stoch_simul that is interpreted as model(linear). See point 2 at https://forum.dynare.org/t/cee2005-model-and-questions/14547

I find this syntax confusing an unintuitive as it works completely different from the loglinear option. I would suggest to remove it.