evalue_backsubstitute: add required modulo constraints

evalue_backsubstitute: add required modulo constraints

evalue_backsubstitute would already add equalities corresponding
to the mapping from new parameters to old, but failed to
add also the corresponding modulo constraints.
Instead, the caller would ensure that that no such constraints
were needed by adding them itself beforehand.  This (old) code
can be removed now that this is handled by evalue_backsubstitute.