Disable smart strings in lxml XPath evaluations

lxml XPath string results are "smart" by default. They have a `getparent()` method to know their parent `Element` (http://lxml.de/xpathxslt.html#xpath-return-values).

> There are certain cases where the smart string behaviour is undesirable. For example, it means that the tree will be kept alive by the string, which may have a considerable memory impact in the case that the string value is the only thing in the tree that is actually of interest. For these cases, you can deactivate the parental relationship using the keyword argument smart_strings.

This functionality is not used in Scrapy selectors.