recurrence: add a function to check equality of recurrences

The gcal_recurrence_is_equal function checks if two recurrences are
equal or not. When a user is trying to modify the recurrence-rules
of an already recurring event, this function tells us if the
recurrence-dialog should pop up or not.

This patch also removes some unused code.

https://bugzilla.gnome.org/show_bug.cgi?id=782755