[QML] Correctly watch for unregistration

Summary:
We wont get the serviceUnregistered signal if only the
WatchForRegistration flag is set.

Remove the unused service name from the balooStarted method.

Test Plan:

1. start kinfocenter
2. balooctl disable

Reviewers: Baloo, ngraham, davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28288