chore: remove merging logic

[qued]

Merging logic has been moved to unstructured for a long time now, this PR removes the leftover logic here.

---

Note

Removes obsolete layout merge logic and related tests, drops unused MERGED source, and bumps version to 1.0.10 with changelog update.

• Core:
  • Remove merge_inferred_layout_with_extracted_layout from unstructured_inference/inference/layoutelement.py and associated unused imports.
• Tests:
  • Delete test_merge_inferred_layout_with_extracted_layout and related imports in test_unstructured_inference/test_elements.py.
• Constants:
  • Drop unused Source.MERGED from unstructured_inference/constants.py.
• Versioning/Docs:
  • Bump __version__ to 1.0.10 and update CHANGELOG.md.

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[ajjimeno]

LGTM!