code: static String getSqlDropColumnDefault(EntityType entityType, Attribute attr) {
      return "ALTER TABLE "
          + getTableName(entityType)
          + " ALTER COLUMN "
          + getColumnName(attr)
          + " DROP DEFAULT";
    }
docs: Returns SQL string to remove the default value from an existing column.
  
  @param entityType entity meta data
  @param attr attribute
  @return SQL string