[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
Latest changes
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* table of `all latest changes`_
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.. _all latest changes: ../view?rql=Any+M%2CX+WHERE+X+modification_date+M+ORDERBY+M+DESC+LIMIT+30