Undo fallback in abstract_dynamics.py

[justinjfu]

The fallback trajectory causes bugs with sim agents ignoring collisions and driving with constant velocities as seen in #52. This is because when the sim agent outputs invalid actions, the fallback takes over and controls the agent with constant velocity.

This PR makes the fallback behavior optional and turns it off by default.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.