Remove deprecated #write_graph method.

Author: gkellogg

lib/json/ld/writer.rb

@@ -108,16 +108,6 @@ def initialize(output = $stdout, options = {}, &block)
       end
     end
 
-    ##
-    # Write whole graph
-    #
-    # @param  [Graph] graph
-    # @return [void]
-    def write_graph(graph)
-      debug {"Add graph #{graph.inspect}"}
-      @repo = graph
-    end
-
     ##
     # Adds a statement to be serialized
     # @param  [RDF::Statement] statement