Changing group prototype name recreates prototype instead of updating it

Steps to reproduce:

1. create host discovery with group prototype
2. discover one or more hosts
3. observe that a group with group_discovery link is created
4. change group prototype name

Result:
The old group_prototype with discovered group and its group_discovery link is removed and a new group_prototype is created.

Expected:
The existing group_prototype is updated. The discovered group with group_discovery link stays the same until next lld rule is processed, when the discovered group with group_discovery link is updated.