KService: remove broken concept of global sycoca database

Summary:
The implementation was very bad (it would delete your
~/.local/share/applications due to a porting bug!), and the usefulness
is close to zero, copying a sycoca file on startup (for kiosk scenarios)
does the same. See discussion on k-f-d for more details.

Test Plan: Builds

Reviewers: sitter

Reviewed By: sitter

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22687