Remove D8 taxonomy term reference field handler

[jonathanjfshaw]

According to core's change record https://www.drupal.org/node/2456869:

"Drupal 8 has a generic Entity reference field included in core, and, until Drupal 8.0.0-beta10, it also had the old Taxonomy reference field from Drupal 7. In order to reduce code duplication and improve the developer and site-building experience, the Taxonomy reference field has been removed."

Let's remove the D8 taxonomy reference field handler from the driver; it's unnecessary maintenance.