[Bugfix/Refactor]--Acquire render context on object removal; Cleanup superfluous add objects

[jturner65]

Motivation and Context

This small PR adds the acquisition of the render context when objects are deleted in PhysicsManager, as well as when Articulated Objects are created (necessary now since we no longer pass through Simulator to instantiate these objects). A few redundant "PhysicsManager::addObject" overloads were also removed.

How Has This Been Tested

C++ and python tests locally pass.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.