remove stale _root_element code
remove stale _root_element code

This was holding a reference to the document and not allowing it to be cleaned up.

closes #97

Visya/libxmljs,rc0x03/libxmljs,Konsus/libxml,Visya/libxmljs,libxmljs/libxmljs,hildjj/libxmljs,CodeFridge/libxmljs_mem,rchipka/libxmljs,Konsus/libxml,CodeFridge/libxmljs_mem,rc0x03/libxmljs,improvisio-software/libxmljs,rc0x03/libxmljs,Visya/libxmljs,polotek/libxmljs,sergeche/libxmljs,apeace/libxmljs,libxmljs/libxmljs,sergeche/libxmljs,CodeFridge/libxmljs_mem,rchipka/libxmljs,apeace/libxmljs,gagern/libxmljs,Konsus/libxml,improvisio-software/libxmljs,gagern/libxmljs,hildjj/libxmljs,libxmljs/libxmljs,polotek/libxmljs,hildjj/libxmljs,improvisio-software/libxmljs,rchipka/libxmljs,polotek/libxmljs,sergeche/libxmljs,gagern/libxmljs