Remove JsonDB specific tests and plug-ins

Task-number: QTBUG-31741

JsonDB is basically dead. Until quite recently it didn't even build.
There is no platform which can use those code lines.

Change-Id: I071eb8b78165dcb07d4432af1f49f800cee595dc
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Remove jsondb related docs and example code

Task-number: QTBUG-31741

Change-Id: I0ee10f459d79607f06d37ae7317c2d54a8d7d56a
Reviewed-by: Lars Knoll <lars.knoll@digia.com>