remove server_extensions mixin

src/legacy/server/kbn_server.d.ts

@@ -73,10 +73,6 @@ declare module 'hapi' {
     ) => void;
     getInjectedUiAppVars: (pluginName: string) => { [key: string]: any };
     getUiNavLinks(): Array<{ _id: string }>;
-    addMemoizedFactoryToRequest: (
-      name: string,
-      factoryFn: (request: Request) => Record<string, any>
-    ) => void;
     logWithMetadata: (tags: string[], message: string, meta: Record<string, any>) => void;
     newPlatform: KbnServer['newPlatform'];
   }

src/legacy/server/kbn_server.js

@@ -34,7 +34,6 @@ import configCompleteMixin from './config/complete';
 import { optimizeMixin } from '../../optimize';
 import * as Plugins from './plugins';
 import { savedObjectsMixin } from './saved_objects/saved_objects_mixin';
-import { serverExtensionsMixin } from './server_extensions';
 import { uiMixin } from '../ui';
 import { i18nMixin } from './i18n';
 
@@ -91,8 +90,6 @@ export default class KbnServer {
 
         coreMixin,
 
-        // adds methods for extending this.server
-        serverExtensionsMixin,
         loggingMixin,
         warningsMixin,
         statusMixin,