Remove obsolete load method

Summary:
The default implementation of load in IncidenceEditor already
does what we did here: nothing.

Test Plan:
Compiles. I also played around with changing some events randomly,
both categories and/or other changes, also this diff should effectively
be a no-op.

Reviewers: KDE PIM, dvratil

Reviewed By: KDE PIM, dvratil

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D7552