refactor: remove redundant Client#rest and obsolete RESTManager#destroy (#2022)

src/rest/RESTManager.js

@@ -32,12 +32,6 @@ class RESTManager {
     return Endpoints.CDN(this.client.options.http.cdn);
   }
 
-  destroy() {
-    for (const handler of Object.values(this.handlers)) {
-      if (handler.destroy) handler.destroy();
-    }
-  }
-
   push(handler, apiRequest) {
     return new Promise((resolve, reject) => {
       handler.push({