Remove drop_null_columns param from doc reference.

Author: mashhurs

docs/index.asciidoc

@@ -387,7 +387,6 @@ Accepted options:
 |===
 |Option name |Default value | Description
 
-|`drop_null_columns` |`false` | Requests {es} to filter out `null` columns
 |`named_params` |[] | List of named parameters and their matches used in the `query`
 |===