sql_io.py: cleanup_table(): Use sql.table_pkey_col() instead of sql.pkey_col() so that only an actual pkey column is removed from the list of columns to clean. This fixes a bug where the first column in the table was not cleaned up if there was no pkey. Note that this bug only affected newly re-created staging tables, because staging tables previously had a special row_num pkey column added if they did not already have a pkey. The row_num column is now added by column-based import instead.