glTF exporter: remove proxy management

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/exp/gltf2_blender_gather.py

...nder/exp/gltf2_blender_gather_animation_channel_target.py

...f2/blender/exp/gltf2_blender_gather_animation_channels.py

...r/exp/gltf2_blender_gather_animation_sampler_keyframes.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_nodes.py