Revert "in createItemSyncInstanceIfMissing(), warn but don't assert if"

This reverts commit c5a8ffd525c320eff45734810fd5449d7af2e9c3.

ASSERTs often exist for a good reason. In this case:
<vkrause> breaks the assumption of just one task being processed at the same time.

I'm looking into fixing one of the cases that triggers this assert, which
is a better solution than just silently ignoring the issue...

CCMAIL: allen@kde.org
(cherry picked from commit 84c7c4882f886512d196c606a2750884cd47f0f8)