[MLIR][Presburger] Move IntegerPolyhedron::reset to FlatAffineConstraints::reset

This patch moves IntegerPolyhedron::reset to FlatAffineConstraints::reset. This
function is not required in IntegerPolyhedron and creates ambiguity while
shifting implementations to IntegerRelation.

This patch is part of a series of patches to introduce relations in Presburger
library.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D120628