(icalendar-export-region): Remove unused vars 'start', 'sum', and
'contents-n-summary'.
(icalendar--parse-summary-and-rest): Remove unused var 'pos-sum' and 'sum'.
(icalendar--convert-float-to-ical): Use calendar-let* to bind vars
'date' and 'entry'.
(icalendar--convert-non-recurring-all-day-to-diary):
Remove 'event' arg. Update callers.
(icalendar--convert-non-recurring-not-all-day-to-diary): Remove 'event'
and 'dtend-dec' args. Update callers.