Note: This is typically only required for scenarios required advanced manual XML manipulation.
Use a Data Process step with Search/Replace processing step with the following regular expression configuration:
Text to Find: <\?xml.*\?>
Replace With: (no value/blank)
Note: The question mark character is a reserved metacharacter in Java regular expressions and must be escaped with a backlash (\). The .* notation is a wildcard that will match any characters before the declaration closing tag.
The full list of metacharacters are:
See http://docs.oracle.com/javase/tutorial/essential/regex/literals.html for more information.