Fix spurious error message for ramsey_model with ramsey_policy
The file dynare_forum_2102.mod generates
ERROR: dynare_forum_2102.mod: line 258, cols 1-78: ramsey_policy: the 'planner_discount' option cannot be used when the 'optimal_policy_discount_factor' parameter is explicitly declared.
but there is no object optimal_policy_discount_factor
present in the model or mentioned in the manual.