Get rid of mostly unused filePathToStat overload

Summary:
It is only used in one place to retrieve the device ID, which can also
be retrieved from the fileId.

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18833