refactor(infra): remove old plugin system (#5411)

plugin system need redesign

tests/kit/utils/load-page.ts

@@ -6,10 +6,6 @@ export async function openHomePage(page: Page) {
   await page.goto(coreUrl);
 }
 
-export async function openPluginPage(page: Page) {
-  await page.goto(`${coreUrl}/_plugin/index.html`);
-}
-
 export async function open404Page(page: Page) {
   await page.goto(`${coreUrl}/404`);
 }