Remove duplicated code for rating property
Summary:
The rating can be handled like any other integer valued property, i.e.
using the generic property<->prefix mapping.
Test Plan: $> baloosearch 'rating>=5'
Reviewers: Baloo, Frameworks, ngraham, poboiko
Reviewed By: Baloo, ngraham
Subscribers: kde-frameworks-devel
Tags: Frameworks, Baloo
Differential Revision: https://phabricator.kde.org/D17274