chore(isthmus): remove dead code

[bestbeforetoday]

Since the use of DdlSqlToRelConverter was added to SubstraitSqlToCalcite in commit 260a1c4, the original invocation of SqlToRelConverter that was left as a fallback when it returns null is never called. The reason is that DdlSqlToRelConverter itself already falls back to calling SqlToRelConverter so it never returns null. This means that removeRedundantProjects is never applied to the Rel structure since this only occurs in the uncalled fallback path. All tests continue to pass, which indicates that this optimization is unnecessary.

If it is required in the future, it might be better written as follows to provide flexibility in the rules applied:

static RelRoot applyPlannerRules(RelRoot root, RelOptRule... rules) {
  HepProgramBuilder builder = HepProgram.builder();
  for (RelOptRule rule : rules) {
    builder.addRuleInstance(rule);
  }

  HepPlanner planner = new HepPlanner(builder.build());
  planner.setRoot(root.rel);

  RelNode optimizedExpression = planner.findBestExp();

  return root.withRel(optimizedExpression);
}