Remove screen element "Screen"

Smooth migration from screens to dashboards requires some changes. Existing screen element "Screen" cannot be migrated to Dashboards, it should be removed.

Zabbix acceptance

1. Existing screen element "Screen" must not be supported anymore
   1. A database patch must be created to remove it from all existing screens
2. Documentation must be updated

Nonfunctional requirements

1. N/A

Use cases

1. I want to have only dashboards that should incorporate most of functionality of screens

Decision made

1. There will be no support of the "Screen" element in Dashboards. It is a functional regression, which will hopefully affect only a tiny fraction of Zabbix users.