Remove extra yarn PnP example test

[ijjk]

Since this test seems to be failing here and there and we already have a TypeScript PnP test this removes this specific fixture

x-ref: https://github.com/vercel/next.js/runs/6197090560?check_suite_focus=true
x-ref: #36516 (comment)

[ijjk]

Failing test suites

Commit: 8c3b62a

yarn testheadless test/integration/custom-routes/test/index.test.js

• Custom routes > serverless mode > should handle external beforeFiles rewrite correctly

Expand output

● Custom routes › serverless mode › should handle external beforeFiles rewrite correctly

expect(received).toBe(expected) // Object.is equality

Expected: 200
Received: 500

  89 |       console.error('Invalid response', html)
  90 |     }
> 91 |     expect(res.status).toBe(200)
     |                        ^
  92 |     expect(html).toContain('Vercel')
  93 |
  94 |     const browser = await webdriver(appPort, '/nav')

  at Object.<anonymous> (integration/custom-routes/test/index.test.js:91:24)

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