When removing a "parent" hosts (for example by discovery actions) server does not remove its corresponding host prototypes and hosts created from them.

For example we have host host.A and a discovery rule with host prototype host.PROTO.
Two hosts host.A.1 and host.A.2 are discovered by this rule.
In this case removing host.A must also remove host prototype host.PROTO and hosts host.A.1,host.A.2.