Change subject-level mode DELETE response type from String to ModeGet…

[xiaoyali97]

…Response

What?
Change the subject-level Mode DELETE API reponse type, added in #1780

Test
Done manual test. The resource is also covered by existing unit tests.

 % curl -X PUT -H "Content-Type: application/vnd.schemaregistry.v1+json" \   
    --data '{"mode": "READONLY"}' \     
    http://localhost:8081/mode/Kafka-value
{"mode":"READONLY"}%                                                                                                                                                                           
 % curl -X DELETE -H "Content-Type: application/vnd.schemaregistry.v1+json" \
    http://localhost:8081/mode/Kafka-value
{"mode":"READONLY"}%   

[rayokota]

LGTM