Return the expected response when webdriver requests a session deletion.
This fixes a mistake that I have to deal with every time I try to use our webdriver server implementation for something new. We don't have any automated tests for this functionality yet (#21370) but I want to stop stumbling over it.
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- [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 ¯\\\_(ツ)\_/¯
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