Optimize Baloo::File copy assign operator, fix Baloo::File::load(url)
There is no need to delete and create a new Private on assignment, it is
sufficient to reassign its members.
File::load(url) resets the url, afterwards he old corresponding property
map is no longer valid, clear it.
Test Plan: ctest
Reviewed By: Baloo, ngraham
Differential Revision: https://phabricator.kde.org/D18877