Remove Bullet from the Bakge library
The reasons for this are pretty straightforward. Bullet is a large and
diverse piece of software that would be foolish to abstract over. Users
should be able to set up their physics worlds how they want it. It will be
very straightforward to extract mesh data from bakge::Mesh objects, which
can then be used to create colliders. And all Shapes classes will provide
the necessary API to extract dimension info used to create primitive
shapes in Bullet or any other physics library.

This has taken some thought but I feel it is best to leave the choice of
physics engine up to the end-user. The GDK engine will still use Bullet
for its physics, however.