MDL-67353 course: move form tearDown() to reset_all_data()

We are already cleaning all sort of caches, statics, singletons
there and it's better to ensure they are always reset to avoid
myterious failures @ distance.

customfield/tests/data_controller_test.php

@@ -34,14 +34,6 @@ use core_customfield\data_controller;
  */
 class core_customfield_data_controller_testcase extends advanced_testcase {
 
-    /**
-     * Tear down to reset the singleton after each test.
-     */
-    public function tearDown() {
-        core_course\customfield\course_handler::reset_after_test();
-        parent::tearDown();
-    }
-
     /**
      * Get generator.
      *