gnus-icalendar: Missing attendees are not REQ-PARTICIPANT

gnus-icalendar: Missing attendees are not REQ-PARTICIPANT

* lisp/gnus/gnus-icalendar.el (gnus-icalendar-event--get-attendee-names)
(gnus-icalendar-event-from-ical): Do not default to REQ-PARTICIPANT
if the user was not found as an attendee.
* test/lisp/gnus/gnus-icalendar-tests.el
(gnus-icalendary-weekly-byday): Remove the ROLE property to test
that it correctly defaults to REQ-PARTICIPANT.  The case where the
user is not listed is covered by gnus-icalendar-parse already
(bug#50749).