Remove numpy from nextjs-flask to fix broken builds (#947)

### Description
pip fails to install this dependency on Python 3.12

* Stacktrace:
https://gist.github.com/sup/f0ff6eb90c03fbe7bd4329b0202dbbb8
* Issue: #946

While fixing the root cause of the error is certainly one way of going
about it, numpy is not even used for the hello-world Flask app this
ships with. To simplify things, this commit removes numpy as a
dependency entirely.

### Demo URL

https://automaton-seven.vercel.app/

### Type of Change

- [ ] New Example
- [x] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### New Example Checklist
Not applicable

- [ ] 🛫 `npm run new-example` was used to create the example
- [ ] 📚 The template wasn't used but I carefuly read the [Adding a new
example](https://github.com/vercel/examples#adding-a-new-example) steps
and implemented them in the example
- [ ] 📱 Is it responsive? Are mobile and tablets considered?

Author: sup

python/nextjs-flask/requirements.txt

@@ -1,2 +1 @@
-numpy==1.24.3
 Flask==3.0.3