Revert "Unrevert "Fix Gating to use grades API, instead of its own calculations""

...s/course_api/blocks/transformers/tests/test_milestones.py

lms/djangoapps/gating/api.py

lms/djangoapps/gating/signals.py

lms/djangoapps/gating/tests/test_api.py

lms/djangoapps/gating/tests/test_signals.py

lms/djangoapps/grades/module_grades.py

lms/djangoapps/grades/new/subsection_grade.py

lms/djangoapps/grades/tasks.py

lms/djangoapps/grades/tests/test_grades.py

lms/djangoapps/grades/tests/test_tasks.py