Refactor/cleanup keyboard handling in molecule container

[#42312179]

- remove unused older keyboard handling code

- fix subsequent calls to model_player.play() when return/enter
key pressed when already playing -- this caused model to speedup,
presumably because multiple callbacks were being scheduled.

Speedup only easily noticeable on model where computational
load for model engine and view are completed in a fraction
of the time the default 60Hz-based animation frame callback
leaves available.

example: interactives/samples/2-simple-gas.json