Revert "[MLIR][Presburger] LexSimplex::addEquality: add equalities as fixed columns"

This reverts commit 5630143a. The
implementation in this commit was incorrect. Also, handling this representation
of equalities in the upcoming support for symbolic lexicographic minimization
makes that patch much more complex. It will be easier to review that without
this representaiton and then reintroduce the fixed column representation
later, hence the revert rather than a bug fix.