Remove unused macOS defaults placeholder script

[southpawriter02]

Removed install/set-macos-defaults.sh as it was an unused placeholder. The logic for macOS defaults is already implemented in install/04-macos-system-settings.sh and install/11-defaults-and-additional-configuration.sh.

Verified that the removal does not break the installation process by running ./install.sh --dry-run. No references to the file exist in the codebase other than in documentation/comments which refer to it as a placeholder to be refactored/removed.

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PR created automatically by Jules for task 13834354042802739861 started by @southpawriter02

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