Remove unused SubrangeBound(s) classes (#221)

Remove unused SubrangeBound(s) classes (#221)

As discussed in pull request #220, commit
360d6ee8d3cdd5ddb47c328043eaed3a107b8db1 mostly got rid of SubrangeBound(s).
There were still a few mentions of it left in the code, most of them commented
out. The occurrences in expr.i and expr_manager.i, however, created issues with
the Python wrapper. This commit removes the SubrangeBound(s) implementation and
other leftovers.