[migration] don't handle data deletion anymore on schema changes In most cases when we want to drop some entity/relation type, we don't care whether hooks are called on their deletion. There is even low chances that some hooks still exists, based on an old version of the schema. Last but not least, this is horribly inefficient. So this should be clearly documented and handled by application's programmer if desired. This patch removes unnecessary deletion (because table or column will be later dropped) and reimplements the case of partial deletion (only one relation definition among several, hence the database structure isn't modified) using sql. Only one test regarding deletion of inlined relation def is added as other cases seem to be covered by existing tests. Closes #7023315

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