ox-icalendar: Speed up `org-agenda-write' process
* lisp/ox-icalendar.el (org-icalendar-create-uid): Remove unused
optional argument.
(org-icalendar--combine-files): Change signature. Simplify process.
(org-icalendar-combine-agenda-files): Apply signature change. Do not
check anymore ICALENDAR-MARK property.
(org-icalendar-entry): Do not check anymore ICALENDAR-MARK property.
(org-icalendar-export-to-ics): Comply to comments.
(org-icalendar-export-current-agenda): Rewrite function.
* lisp/org-agenda.el (org-agenda-write): Update docstring.
Instead of parsing every agenda before picking up needed entries, copy
these entries in a temporary buffer, then export it.