Remove old testlab code

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Testlab was never released, and the code has been sitting there for years unused. Clean it up, along with the few references to it.

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[gemini-code-assist]

Code Review

This pull request completely removes the Firebase Test Lab (testlab) module from the repository. This includes deleting all platform-specific source files (Android, iOS, Desktop, and common), headers, FlatBuffers schemas, build configurations (CMakeLists.txt), and dependency declarations in Gradle and GHA integration testing configurations. I have no feedback to provide as there are no review comments.