If Outlook created the calendar folder in the PST, then you won't be able to
delete it with Outlook.  If that PST was ever a delivery location, then it
will have the undeletable standard folders in addition to any you created.

One thing I've done before is to create another PST and then drag to it
those calendars I wish to keep, after which I close the old PST, thus giving
me a new set of folders with only that data I wish to keep active.

By default Outlook uses the primary calendar for all appointments sent
to you. If you are not using the primary calendar at all, then why
have a second calendar? The best thing to do is to set your calendar
view to categories, select all appointments, right-click and move them
back to the primary calendar. In my experience a second calendar will
not function the same as the primary. Your meeting requests will go
the the primary by default. But Outlook does not allow you to delete
or rename the primary calendar.

There is a utility you can use to delete it, but I don't know what
affect that will have on your calendar behavior or if it is a wise
thing to do. The only time I have used it is to delete and the re-
create the primary calendar to resolve a corrupted calendar issue.