Avoid indirection when inserting basic data

Summary:
When setFileItem() is called, m_fileItems.size() is always 1, for
setFileItems() there are always multiple elements, thus it is known which
branch of insertBasicData() will be taken. Just call the corresponding
target method directly, and remove the unused private insertBasicData().

Test Plan: ctest

Reviewers: #baloo, #frameworks, astippich, ngraham

Reviewed By: #baloo, ngraham

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D20477