Remove Drupal CI testing configurations for Drupal 8

Drupal 8 is no longer supported. Stop testing against Drupal 8 so we can work on getting the Drupal 9 tests working.

Note, Entity API has a drupalci.yml file that makes automated tests fail on deprecations. This means that if the codebase is using a deprecated function/method/class/etc. then the tests will fail. Because a lot of things are deprecated in D9, and because the list of deprecations keeps growing, it is impossible to pass the D8 and the D9 tests at the same time. Since D8 is EOL, the focus should now be on making the D9 tests pass.