Fix AkonadiStorageTestBase::shouldNotifyItemTagRemoved

Summary:
Both a task and a note were associated to "philosophy-tag", which made
the test fail (when the note showed up first in the list of tagItems).
We don't need the note anyhow, so untag it.

Test Plan: That method passes now. Down to 1 failure.

Reviewers: ervin

Reviewed By: ervin

Subscribers: Zanshin

Differential Revision: https://phabricator.kde.org/D18691