OpObserverImpl::onDelete() is always called with engaged deletedDoc value for findAndModify deletes.

• https://github.com/mongodb/mongo/blob/49f6fcebd3c4a97abcd05c862ad1204eed9d66ef/src/mongo/db/op_observer_impl.cpp#L671-L673
• https://github.com/mongodb/mongo/blob/49f6fcebd3c4a97abcd05c862ad1204eed9d66ef/src/mongo/db/catalog/collection_impl.cpp#L1062-L1073
• https://github.com/mongodb/mongo/blob/49f6fcebd3c4a97abcd05c862ad1204eed9d66ef/src/mongo/db/exec/delete.cpp#L204-L205
• https://github.com/mongodb/mongo/blob/49f6fcebd3c4a97abcd05c862ad1204eed9d66ef/src/mongo/db/commands/find_and_modify.cpp#L189

The consequences of this are two-fold:

1. findAndModify deletes within a multi-statement transaction write preImage no-op oplog entries, even when the recordPreImages collection option is set to false (the default). This leads to write amplification akin to SERVER-45442.
2. preImage no-oplog entries from multi-statement transaction deletes do not include session information. This leads donor shards in chunk migrations to crash when parsing them.