Remove the line, or remove that fragment from the line? Removing the line renders the structure incomplete, as you will be left with a closing /AutoData without a corresponding opening tag. That could be fixed by removing the closing tag too, of course, but then again that might violate the DTD. Let's assume you mean "remove the fragment".
sed 's% xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"%%' old.xml >new.xml