Allowing KAMD to run without a database backend

Summary:
On systems without the support for shared memory which sqlite wal
can not run on, just disable the sqlite plugin instead of killing KAMD.

Also, before we have a proper way to handle database corruption,
this will at least allow KAMD to be started in this case.

Reviewers: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12577