Fix SQL syntax error in findOrphanedItems()

Summary:
SQL UPDATE does not support full field name:

PimItemTable.collectionId = :0

vs

collectionId = :0

in its SET part.

Reviewers: KDE PIM, dvratil

Reviewed By: KDE PIM, dvratil

Subscribers: dvratil, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D17876