[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