Squeezebox remove deprecated sync and unsync services

[rajlaud]

Breaking change

We previously gave notice that the squeezebox platform had deprecated the squeezebox_sync and squeezebox_unsync services and the sync_group attribute in favor of the newer "join" framework common to all media players. See #70962. These have now been removed.

Proposed change

Unfortunately, I forgot to remove these deprecated services and the attribute after two releases. This PR removes them.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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• This PR fixes or closes issue: fixes #
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