Allow omission of course_key in call to CourseWaffleFlag.is_enabled

This effectively evaluates the flag outside of the context of a course.
This was previously available through `.is_enabled_without_course_context`,
which has been removed in favor of simply `is_enabled()`.

This was done to make the CourseWaffleFlag interface more uniform with
that of WaffleFlag and ExperimentWaffleFlag and eliminate unecessary
branching when handling CourseWaffleFlags.