Remove document from MTimeDB/DocumentTimeDB even when timestamp is 0

Summary:
Commit 628daced19b8 ("allow ctime/mtime == 0") removed the asserts
checking for 0 (anyway only effective for debug builds), but left
the code for the timestamp removal unchanged.

Test Plan:
$> touch -d '@0' xxx.txt
$> balooshow 'mtime<1980-01-01'

-> xxx.txt shows up

$> touch xxx.txt
$> balooshow 'mtime<1980-01-01'

-> xxx.txt no longer shown

Reviewers: Baloo, Frameworks, ngraham, poboiko, astippich

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20280