Remove XEmacs compat code from icalendar.el (Bug#37816)

Remove XEmacs compat code from icalendar.el (Bug#37816)

* lisp/calendar/icalendar.el (icalendar--convert-string-for-export)
(icalendar--convert-string-for-import)
(icalendar--parse-summary-and-rest)
(icalendar--convert-ordinary-to-ical)
(icalendar--convert-weekly-to-ical)
(icalendar--convert-yearly-to-ical)
(icalendar--convert-block-to-ical)
(icalendar--convert-cyclic-to-ical)
(icalendar--convert-anniversary-to-ical)
(icalendar--format-ical-event)
(icalendar--convert-recurring-to-diary): Remove XEmacs compat code.
(icalendar--rris): Declare obsolete.