[R/pugi] provide xml_rm_child()

[JanMarvin]

Use cases

> xml_node <- "<a><c>1</c><c>2</c></a>"
> xml_child <- "c"
> 
> # remove all c nodes
> xml_rm_child(xml_node, xml_child, which = 0)
[1] "<a/>"
> # remove only the first c node
> xml_rm_child(xml_node, xml_child, which = 1)
[1] "<a><c>2</c></a>"
> # remove only the second c node
> xml_rm_child(xml_node, xml_child, which = 2)
[1] "<a><c>1</c></a>"

[JanMarvin]

I'll merge this, but the output formats might be updated in a follow up commit.