make clean leaves a number of zz_* generated files around

### What happened?

After running `make clean`, checking for left over generated files with `find . -name "zz_*.go"` shows a list of files that were not cleaned up. Is this on purpose or should `make clean` clean up all generated files?

### How can we reproduce it?

Checkout an upjet generated provider that has resources.

`make clean`
`find . -name "zz_*.go"`