[kded/baloosearchmodule] Remove useless qDebug messages

Summary:
Those messages are not informative anyway; they are categorized as "kdeinit5";
and they tend to spam logs during indexing with filenames of indexed files.

Test Plan: make

Reviewers: Baloo, bruns, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D22942