[Scheduler] Remove extraneous set of content indexing flag

Summary:
The content indexing flag is already set by the BasicIndexingJob::index()
in ModifiedFileIndexer::run() executed previously. Instead of doing
another write transaction just update it when updating the other DBs
in replaceDocument(), like it is already done in addDocument().

Also, the flag was even set when the config only specifies BasicIndexing,
i.e. content indexing was always done.

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16253