Revert "MDL-34894 course ws unit tests: fix windows problem."

This reverts commit 954c272e548c0578fde9d1bc0979ac46b72bc3b9.

With MDL-35714, each test gets gc_collect_cycles() called, so
no need to call it "manually".

course/tests/externallib_test.php

@@ -580,6 +580,5 @@ class core_course_external_testcase extends externallib_advanced_testcase {
 
         // Check that the course has been duplicated.
         $this->assertEquals($newcourse['shortname'], $duplicate['shortname']);
-        gc_collect_cycles();
     }
 }