🧹 Remove unused using directives in models

[antarr]

🎯 What: Removed unused using System.Runtime.Serialization; directives from Measuregrp.cs and WithingsBody.cs.
💡 Why: These directives were not used (no DataContract or DataMember attributes present), and removing them improves code maintainability and readability.
✅ Verification: Verified that the project builds and all unit tests pass on .NET 8.0 and .NET 10.0.
✨ Result: Cleaner codebase with reduced noise in model files.

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PR created automatically by Jules for task 12050682321863584092 started by @antarr

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