[hooks/syncschema] Turn DropColumnOp into a data operation While writing a test for a migration issue where "NOT NULL" was added to an inlined relation column, I ran into a case where the same column was deleted twice, which of course results in an error. This is because the "ecrit_par" relation used by the test is ambiguous. I've hence turned the DropColumnOp into a data operation to ensure we'll delete the column only once in such case. Related to #6211101

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