When deleting tax type, it should delete its children tax rate and rate amounts

Problem/Motivation

When deleting tax type, its children (tax rates) will get orphaned.

Same thing with tax rates. When tax rate gets deleted, its children (tax rate amounts) will get orphaned.

Proposed resolution

On \Drupal\commerce_tax\Entity\TaxType::postDelete() and \Drupal\commerce_tax\Entity\TaxRate::postDelete() call its children and delete them.