Move internal helper from BasicIndexingJob to anonymous namespace

Summary:
Also remove the return value, as it not evaluated, and not even useful -
we are not interested in the presence of tags, but if the tags where
modified. The latter can only be determined from the write transaction
(Read-modify-check-write).

Reviewers: Baloo, Frameworks, astippich, ngraham

Reviewed By: astippich

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D17324