DGS-1328 Add DELETE API for subject config and subject mode

[xiaoyali97]

Done manual test, will update system tests later.

Example usage:

 % curl -X GET http://localhost:8081/config
{"compatibilityLevel":"BACKWARD"}%  
 % curl -X PUT -H "Content-Type: application/vnd.schemaregistry.v1+json" --data '{"compatibility": "FORWARD"}' http://localhost:8081/config/Kafka-key 
{"compatibility":"FORWARD"}%                                                                                                                                                                  
 % curl -X GET http://localhost:8081/config/Kafka-key                                                                                                
{"compatibilityLevel":"FORWARD"}%                                                                                                                                                              
 % curl -X DELETE -H "Content-Type: application/vnd.schemaregistry.v1+json" http://localhost:8081/config/Kafka-key
{"compatibilityLevel":"FORWARD"}%                                                                                                                                                              
 % curl -X GET http://localhost:8081/config/Kafka-key                                                             
{"error_code":40401,"message":"Subject 'Kafka-key' not found."}%                                                                                                                               
 % curl -X GET "http://localhost:8081/config/Kafka-key?defaultToGlobal=true"
{"compatibilityLevel":"BACKWARD"}%         

[rayokota]

Thanks @xiaoyali97 , LGTM