Update mujoco-py to MuJoCo 2.1 (#640)

* Update to MuJoCo 2.1.0

- Remove license-related parts of the build process.
- Correctly parse the comments for array sizes based on nmocap.
- Explicitly exclude a few structs from generated wrappers, where
  the structs don't have a corresponding typedef.

Still TODO: Update README.md once a new package is pushed to PyPi.

* Remove MuJoCo key docs from README.md.

As of version 2.1.0, MuJoCo doesn't require an activation key.