Remove GraphSON 2 option in TraversalOpProcessor

There is an option for the TraversalOpProcessor to take a GraphSON 2 string rather than a Bytecode object. Perhaps that is the way it is for legacy reasons - not sure, but being bound to GraphSON 2 seems odd. Need to investigate further, but I would think that we just want to expect a Bytecode object and that's it:

https://github.com/apache/tinkerpop/blob/3.4.11/gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/op/traversal/TraversalOpProcessor.java#L82