fix(study_locus_valiation): ensure the the transQtlColumn is missing

[project-defiant]

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✨ Context

The dataproc job for study_locus_valiation step has failed due to the duplicated isTransQtl column.
This can happen when we read the dataset from the parquet file that will automatically align it to existing schema, resulting in the addition of the empty column, which will result in SchemaValidationError when validating the dataset before writing after joining it with the lookup index containing isTransQtl column.

🛠 What does this PR implement

• Ensure the isTransQtl column is dropped (provided it exists) when running the method StudyLocus.flag_trans_qtls

🙈 Missing

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[vivienho]

All makes sense!