drop unused "extra" field in rule parameters

server/sonar-web/src/main/js/app/types.d.ts

@@ -687,8 +687,6 @@ declare namespace T {
   export type RuleInheritance = 'NONE' | 'INHERITED' | 'OVERRIDES';
 
   export interface RuleParameter {
-    // TODO is this extra really returned?
-    extra?: string;
     defaultValue?: string;
     htmlDesc?: string;
     key: string;

server/sonar-web/src/main/js/apps/coding-rules/components/ActivationFormModal.tsx

@@ -229,7 +229,6 @@ export default class ActivationFormModal extends React.PureComponent<Props, Stat
                     className="note"
                     dangerouslySetInnerHTML={{ __html: param.htmlDesc || '' }}
                   />
-                  {param.extra && <div className="note">{param.extra}</div>}
                 </div>
               ))
             )}

server/sonar-web/src/main/js/apps/coding-rules/components/CustomRuleFormModal.tsx

@@ -329,7 +329,6 @@ export default class CustomRuleFormModal extends React.PureComponent<Props, Stat
           />
         )}
         <div className="note" dangerouslySetInnerHTML={{ __html: param.htmlDesc || '' }} />
-        {param.extra && <div className="note">{param.extra}</div>}
       </td>
     </tr>
   );