Remove `#write_graph` as arcaic, and `#write_statement` to use built-in `#write_statement`, which depends on `#write_triple` but does error checking.
Remove `#write_graph` as arcaic, and `#write_statement` to use built-in `#write_statement`, which depends on `#write_triple` but does error checking.

ruby-rdf/rdf-rdfa,ruby-rdf/rdf-rdfa