Remove usage of register keyword in Extension Modules
- [X] closes #26263
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

ASV results show no difference:

```sh
BENCHMARKS NOT SIGNIFICANTLY CHANGED.
```

As I am fairly certain this keyword was being entirely ignored by the compilers anyway (see issue for reasoning)