Currently when creating a new item it is possible to change the host that will contain the item. Same for applications and web scenarios. This feature is hardly helpful and causes difficulties with implementation of ZBXNEXT-1407.

To remove this feature we'll need to:
1. Forbid creating item and applications when no host is select in the list. That way we'll always now, to which host the object belongs to. For consistency, we should do the same for triggers (it's already done for graphs).
2. Then the "host" field can be completely removed from the application, item and web scenario forms.

This should be done before continuing work on ZBXNEXT-1407.