Revert descendant combinator change

[redapple]

This reverts the change to xpath_descendant_combinator from d86287d so as to keep backwards compatiblity with cssselect-dependent libraries like parsel (see #61 (comment) and subsequent comments)

/descendant:: and /descendant-or-self::*/ are essentially equivalent in practice for the translation when only elements are involved. But for custom pseudo-elements like parsel does with ::text and ::attr(name), which need a bit of hacking of the translated path, it's probably safer to keep the "long" version.

/cc @kmike

[codecov-io]

Current coverage is 95.02% (diff: 100%)

Merging #63 into master will not change coverage

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