Remove Mixpanel Token References

[FrPellissier]

Removes mixpanelToken and MixpanelToken references from EnvironmentConfigurationManager.swift. All other mixpanel SDK usages appear to have been already removed.

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PR created automatically by Jules for task 8329558204035028788 started by @FrPellissier

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[Copilot]

Pull request overview

This PR finishes removing legacy Mixpanel configuration access by deleting the MixpanelToken user storage key and the corresponding EnvironmentConfigurationManager accessor, aligning the code with the already-removed Mixpanel SDK usage.

Changes:

• Removed UserStorageKey.mixpanelToken from EnvironmentConfigurationManager.swift.
• Removed the MixpanelToken computed property from EnvironmentConfigurationManager.

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