Remove unused variables (#3906)

app/javascript/mastodon/middleware/errors.js

@@ -1,13 +1,11 @@
 import { showAlert } from '../actions/alerts';
 
-const defaultSuccessSuffix = 'SUCCESS';
 const defaultFailSuffix = 'FAIL';
 
 export default function errorsMiddleware() {
   return ({ dispatch }) => next => action => {
     if (action.type && !action.skipAlert) {
       const isFail = new RegExp(`${defaultFailSuffix}$`, 'g');
-      const isSuccess = new RegExp(`${defaultSuccessSuffix}$`, 'g');
 
       if (action.type.match(isFail)) {
         if (action.error.response) {