Re-enable Bullet, finalize Migration & Tests

[henningkayser]

Bullet is still disabled at this point even though the port has been completed with moveit/moveit_task_constructor#322. We should remove the BULLET_ENABLE flag by default and always compile it. I'm open to support a build flag that allows configuring what collision environment should be compiled. Things to consider:

• Should we switch to Bullet as default plugin instead of FCL?
• Do we want to move bullet + fcl into a separate package moveit_collision_detection_plugins for faster compile times (moveit_core is a bottle-neck for multi-core systems)

[AndyZe]

I don't think Bullet is ready for prime time yet.

• Seg fault issue: Segfault from the PlanningScene after allocating for Bullet collision detection moveit#2485

• Memory leak issue? https://github.com/ros-planning/moveit_task_constructor/issues/232

• Also, I haven't personally seen any performance improvement.