[nodes] Remove `origin` attribute from `ProgramNode`

[lorisleiva]

This attribute whitelists places a ProgramNode could be creating from — currently set to anchor or shank. This is a limitation for the ecosystem as more and more tools could be generating/retrieving Codama nodes.

We could either make this a plain string to allow any value or simply removing it altogether as I don't think this adds much value to the node.