reader: Use xmlFirstElementChild and xmlNextElementSibling rather than xml::advance_t...

reader: Use xmlFirstElementChild and xmlNextElementSibling rather than xml::advance_to_next_sibling_element

The xml::advance_to_next_sibling_element is redundant with the
xmlNextElementSibling API of libxml.  Similarly, xmlFirstElementChild
is redundant with using xml::advance_to_next_sibling_element on the
xmlNode::children data member.  Let's use the libxml API instead.

	* include/abg-libxml-utils.h (advance_to_next_sibling_element):
	Remove the declaration of this function.
	* src/abg-libxml-utils.cc (go_to_next_sibling_element_or_stay)
	(advance_to_next_sibling_element): Remove definitions of these functions.
	* src/abg-reader.cc (read_translation_unit_from_input)
	(read_elf_needed_from_input, read_corpus_group_from_input): Use xmlNextElementSibling instead
	of xml::advance_to_next_sibling_element.
	(read_corpus_from_input): Likewise.  Also, use
	xmlFirstElementChild instead of
	xml::advance_to_next_sibling_element on the xmlNode::children data
	member.
	(read_corpus_group_from_input): use xmlFirstElementChild instead
	of xml::advance_to_next_sibling_element on the xmlNode::children
	data member.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>