Also remove the obsolete `--cpu` and `--gpu` renderer flags,
which also are no longer used.
Update tests and wptrunner to not pass these flags.
wptrunner changes have been upstreamed in:
- w3c/wptrunner#224
- w3c/wptrunner#226
Index the session past correctly when discarding.
<!-- Please describe your changes on the following line: -->
Oops, indexed from the wrong end when discarding documents in #14312.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because we're not testing document discarding
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14860)
<!-- Reviewable:end -->
