fix camera shudder

[Goober5000]

The enhanced controls and flags in #4552 were intended to allow the camera to shudder in all views, not just the cockpit view. And indeed, the shudder does apply in all other views... except for cutscene cameras, due to the VM_FREECAMERA check in game_render_frame(). So, add a separate check for cutscene cameras when shuddering everywhere. The cutscene camera can be jittered just like the cockpit camera, but the rotation should not be modified or it will produce feedback.

Follow-up to #4552. Also removes an unused variable that was made obsolete in #5656.