Remove Drupal\taxonomy\Tests\HooksTest and taxonomy_test.module

That tests verifies that the load/save/delete hooks are called and does so by adding an additional thing to the term that is stored in an arbitrary table.

- We already have tests for taxonomy hooks in EntityCrudHookTest
- This test predates fieldable terms, this shouldn't be done using a custom table and custom code but by using a field. So it's a bad example.

Let's remove it.