Remove unused DataElement placeholder (#9, #61)

[matthewhorridge]

Summary

src/harmonization_framework/element.py defined DataElement and DEType as placeholders for a domain model that was never finished or adopted. A repo-wide search confirms nothing imports either symbol:

grep -rn \"DataElement\|DEType\" src/ tests/ demo/
src/harmonization_framework/element.py:5:class DEType(Enum):
src/harmonization_framework/element.py:17:class DataElement:
src/harmonization_framework/element.py:24:    element_type: DEType

Only self-references. Rules use plain strings for source/target column names, primitives work on values directly, and the API has no DataElement-shaped endpoints.

The competing ORM-side DataElement that motivated the naming-collision concern in #61 was already removed in 879f49e (Jan 2026) when the database layer was retired — so both the placeholder and the collision it was designed to coexist with are obsolete. The bug in #9 (no __init__, broken __hash__) only matters if the class were used, which it isn't.

Rather than finish an unused abstraction speculatively, delete it. A real DataElement model can be designed when there's a concrete consumer (e.g. ontology integration, RADx Data Dictionary alignment).

Closes #9, #61.

Test plan

• All 167 tests still pass
• No references remain in the repo

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