[Calendars] Auto-subscribed calendars update with enrollments

[pjarosz]

Problem statement:

Currently, auto-subscribed account calendars remain attached and visible to a user even after their enrollment has ended in that sub-account.

Proposed solution:

After a user's enrollment in a course associated with a sub-account has ended, the auto-subscribed calendar is removed from the user's calendar.

User role(s):

admin