This was used by the old compositor of which only the ghostly echoes of
old commits remain. Since it's been unused for quite some time, it is
probably safe to remove it.
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.
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Oops, indexed from the wrong end when discarding documents in #14312.
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- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because we're not testing document discarding
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