Removing simplepbr dependency by default.

[rayanalysis]

As per the discussion on Discord, there are three use cases where we wouldn't expect panda3d-gltf to depend on simplepbr:

• The user isn't expecting to use a separate rendering module
• The user is using custom shaders
• The user is using a separate rendering workflow or module

This is a one-line change which removes the dependency on panda3d-simplepbr by default. It appears that the only part of panda3d-gltf which relies on panda3d-simplepbr is the viewer.py which will prompt the user to install simplepbr if run.

I have tested that this successfully builds using build.

[rayanalysis]

Reopening this PR as I have just verified that not having simplepbr installed does indeed break the loading of gltf files when using panda3d-gltf in a Panda3D Python program. This is a major breaking issue and I believe should be resolved.