Fix performance regression on models with many nested model-local variables without leads/lags

The regression was introduced in commit
84d792bc, when leads/lags were allowed on
model-local variables. The treatment of model-local variables without
leads/lags was negatively impacted. This commit fixes the regression by
restoring the former behaviour for model-local variables without leads/lags.

Reported on https://forum.dynare.org/t/pre-processing-takes-very-long/26865

Ref. dynare#1929