Temporarily disable release notes generation

Release creation fails with this error:
```
HTTP 422: Validation Failed (https://api.github.com/repos/RustPython/RustPython/releases)
body is too long (maximum is 125000 characters)
Error: Process completed with exit code 1.
```

Most likely because there's no previous release to diff of from.

Disabling, getting a green release & enabling back might fix this.