Steps to reproduce:

1. Assign template combining SNMPv2 and Simple items, e.g. Template Module Generic SNMPv2 (with linked Template Module ICMP Ping), to some host, but let the host have Agent and SNMP interface (e.g. by mistake). Save the host.
2. Reopen the host.

Result:
"Remove" link is grey, unable to use it. Simple items from ICMP template are occupying Agent interface, SNMPv2 items SNMP interface.

I am able to move interfaces between types (Agent, SNMP, JMX, IPMI) and save with those changes, but not to remove any of them two.
Expected:
Remove one of redundant interfaces from the host and reassign its items to remaining interface (maybe with warning, maybe with confirmation from the user).

Workaround:

Unlink the template (with item clearing), remove unneeded interface, reassign the template. I think, this is not optimal, and it has taken me some time to realize, what is the cause of this behaviour. (I don't know, why the hosts had unneeded agent interfaces, probably configuration mistake or maybe experimentation...)