[balooctl] clear command: Remove bogus check for documentData, cleanup

Summary:
In case only BasicIndexing is enabled, or when the content indexing
is suspended due to "onBattery" mode, no data is added to the document.

There is no reason to disallow clearing a file which has no data.

Also remove an unused Indexer instance.

Test Plan:
create a file with contentIndexing disabled
try to remove it from the index with balooctl clear <file>

Reviewers: Baloo, Frameworks, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20457