glTF exporter: refactoring: remove no more needed functions after 2.79

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/com/gltf2_blender_math.py

io_scene_gltf2/blender/exp/gltf2_blender_extract.py

...f2/blender/exp/gltf2_blender_gather_animation_samplers.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_joints.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_nodes.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_skins.py