[MTimeDB] Consolidate time interval handling code

Summary:
There are two places where half-bounded time intervals with [<=,<,==,>,>=]
are handled, once in the SearchStore, and once in the MtimeDB itself
(where the latter only has [<=,==,>=]).

Remove the handling from the MtimeDB and only keep the one in the
SearchStore.

Test Plan: ctest -V -R mtimedbtest

Reviewers: Baloo, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28396