Remove long-unused internal plugin system

We are trying to update Express to fix a vulnerability.

We would have to update the plugins as well, but since we are no longer
using the plugin system, we can just delete it instead.

src/node/wsRouter.ts

@@ -1,8 +1,17 @@
 import * as express from "express"
 import * as expressCore from "express-serve-static-core"
 import * as http from "http"
+import * as stream from "stream"
 import Websocket from "ws"
-import * as pluginapi from "../../typings/pluginapi"
+
+export interface WebsocketRequest extends express.Request {
+  ws: stream.Duplex
+  head: Buffer
+}
+
+interface InternalWebsocketRequest extends WebsocketRequest {
+  _ws_handled: boolean
+}
 
 export const handleUpgrade = (app: express.Express, server: http.Server): void => {
   server.on("upgrade", (req, socket, head) => {
@@ -22,9 +31,11 @@ export const handleUpgrade = (app: express.Express, server: http.Server): void =
   })
 }
 
-interface InternalWebsocketRequest extends pluginapi.WebsocketRequest {
-  _ws_handled: boolean
-}
+export type WebSocketHandler = (
+  req: WebsocketRequest,
+  res: express.Response,
+  next: express.NextFunction,
+) => void | Promise<void>
 
 export class WebsocketRouter {
   public readonly router = express.Router()
@@ -36,13 +47,13 @@ export class WebsocketRouter {
    * If the origin header exists it must match the host or the connection will
    * be prevented.
    */
-  public ws(route: expressCore.PathParams, ...handlers: pluginapi.WebSocketHandler[]): void {
+  public ws(route: expressCore.PathParams, ...handlers: WebSocketHandler[]): void {
     this.router.get(
       route,
       ...handlers.map((handler) => {
         const wrapped: express.Handler = (req, res, next) => {
           ;(req as InternalWebsocketRequest)._ws_handled = true
-          return handler(req as pluginapi.WebsocketRequest, res, next)
+          return handler(req as WebsocketRequest, res, next)
         }
         return wrapped
       }),