MDL-50063 unittests: Remove some unnecesary gc_collect_cycles()

Now backup and restore operations free logger resources calling
to the destroy() method. This commit ensures:

1) That gc_collect_cycles() is not used anymore in backup-related tests.
2) That all backup and restore controllers in test do always call
   to the detroy() method.
3) Some unset() calls, needed to make gc_collect_cycles() are not used
   anymore.

admin/tool/uploadcourse/tests/processor_test.php

@@ -36,13 +36,6 @@ require_once($CFG->libdir . '/csvlib.class.php');
  */
 class tool_uploadcourse_processor_testcase extends advanced_testcase {
 
-    /**
-     * Tidy up open files that may be left open.
-     */
-    protected function tearDown() {
-        gc_collect_cycles();
-    }
-
     public function test_basic() {
         global $DB;
         $this->resetAfterTest(true);