Remove automatic column addition for empty scan

Originally a column had to be selected to scan a table, so we always added a "cheap" column to scan.  When we removed this restriction we forgot to remove the code that automatically added a column to an empty table scan. For the Raptor connector, we must still choose a "cheap" column, but this is an internal detail of the Raptor connector.
Add a comment `// last type is sample weight`

I had to look through the file to figure this out