examples: remove `next-env.d.ts` files (#73673)
## Summary
Remove `next-env.d.ts` file from [the
examples](https://github.com/vercel/next.js/tree/canary/examples)
directory because gitignore template includes them.
## Description
At #41041, removed `next-env.d.ts` files.
However, #47241 reintroduced them.
`create-next-app` makes `next-env.d.ts` file in the example directory if
it's typescript.
https://github.com/vercel/next.js/blob/c2078d0c05a03e9fde449a01ff2a10afbdad48bb/packages/create-next-app/create-app.ts#L202-L209
Therefore, `next-env.d.ts` file isn't necessary.
CC: @samcx
### Adding or Updating Examples
- [x] The "examples guidelines" are followed from our contributing doc
https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
- [x] Make sure the linting passes by running `pnpm build && pnpm lint`.
See
https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md
Co-authored-by: JJ Kasper <jj@jjsweb.site>
