Remove unused recurrence assignment operators

Summary:
KCalendarCore::Recurrence::operator= turns out to be broken, this clears
the path to its removal, and avoids accidental use meanwhile.

Reviewers: dvratil, djarvie

Reviewed By: dvratil, djarvie

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D24320