Move the comment about cache clearing to the right place.

It actually doesn't affect program_schoolyear, because that should be
fixed; it only affects getGrade when it is called without a program
(and thus calls current_schoolyear).
Remove unused variable in LSRAssignmentTest
Fix the tests to deal correctly with program-relative schoolyear.
Fix a few other places where YOGFromGrade was used without the
program-relative schoolyear.