Resourcebase docu: don't call changeCommitted from itemRemoved.

Summary:
This attempts to modify an item that was already removed from the DB,
with a "No items found" warning as a result.

Calling changeProcessed() is enough.
That's what most resources do already (knut, maildir, mbox, icaldir,
ical, vcard, kalarmdir, contacts, openxchange...).

It gets harder for resources that have a generic slot though
(e.g. GoogleResource::slotGenericJobFinished or
TomboyNotesResource::onItemChangeCommitted).
The davgroupware resource shows a possible solution for that:
setting and testing modifyJob->property("isRemoval").toBool()

MixedMaildirResource::itemRemovedResult also seems wrong and easily fixable.

Test Plan: FatCRM's unittests no longer show that warning

Reviewers: dvratil, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D13756