Fix check for unused endos in the presence of PAC model-consistent expectations

Since commit 64f55e4a, in the presence of PAC
model-consistent expectations, some endogenous variables are added to the
symbol table at the beginning of ModFile::transformPass(), while their defining
equations are added at a later point.

But commit 7c3f981e has introduced a check that
verifies that all endogenous are used in equations. That check happens after
the above mentioned endogenous are created, but after their defining equations
are added. Hence it fails.

The fix consists in creating those endogenous after the check. Incidently, they
are no longer part of the saved original model, but this is a good thing.