Simularium-Engine -> Octopus Code Cleanup

[ascibisz]

What needs to happen?

• We have some redundant code so that the viewer can communicate with both the simularium-engine and octopus. Once we've fully transitioned to octopus, we can strip out simularium-engine specific code. This includes the following:

• remove the octopus CLI flag and the useOctopus boolean flag, and remove any code that would only be reached when those flags are fals
• simplify the logic for receiving frame data from net connection, since octopus will always only send one frame at a time
• update comments