Broken links in README.md

[pjcozzi]

It would be much appreciated if anyone has cycles to update/remove the following broken links in README.md.

"https://www.opencascade.com/content/cad-assistant ""STEP, IGES, OBJ, PLY, STL"""	404 Not found	Multiple	Dofollow
https://www.assetstore.unity3d.com/#!/content/99793	404 Not found	Unity 2017	Dofollow
https://www.assetstore.unity3d.com/#!/content/40946	404 Not found	Unity 5	Dofollow
https://github.com/emadurandal/GLBoost/blob/master/src/js/middle_level/loader/GLTFLoader.js	404 Not found	loader	Dofollow
https://github.com/AnalyticalGraphicsInc/gltf-utilities	404 Not found	gltf-utilities	Dofollow
https://facebook.github.io/react-vr/	New link -> https://facebook.github.io/react-360/	ReactVR	Dofollow
https://drive.google.com/drive/folders/0BybLPh8Iz_rebnVtbUxLVUswZVE	404 Not found	glTF - Seattle Khronos Group Meetup	Dofollow
https://blog.ybalrid.info/2018/07/16/why-gltf-2-0-is-awesome/	404 Not found	Why glTF 2.0 is awesome!	Dofollow
https://azuremarketplace.microsoft.com/en-us/marketplace/apps/simplygon.simplygon	404 Not found	Simplygon Cloud	Dofollow
https://simplygonclouddoc.simplygon.com/tutorials/firstprocessing.html#asset 404 Not found 

[javagl]

This could become obsolete when KhronosGroup/glTF-Project-Explorer#37 is tackled. I'll try to increase the priority for that.

[javagl]

Also obsolete as of #1796