Clean up RDAP code

For some reason, although a constant was defined for the RDAP response content type, it was being used in one place but not another.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143185157

java/google/registry/rdap/RdapActionBase.java

@@ -115,7 +115,7 @@ public abstract class RdapActionBase implements Runnable {
       if (requestMethod != Action.Method.HEAD) {
         response.setPayload(JSONValue.toJSONString(rdapJson));
       }
-      response.setContentType(MediaType.create("application", "rdap+json"));
+      response.setContentType(RESPONSE_MEDIA_TYPE);
     } catch (HttpException e) {
       setError(e.getResponseCode(), e.getResponseCodeString(), e.getMessage());
     } catch (URISyntaxException | IllegalArgumentException e) {