remove stb submodule

Author: devshgraphicsprogramming

.gitmodules

@@ -39,9 +39,6 @@
 [submodule "3rdparty/bullet3"]
 	path = 3rdparty/bullet3
 	url = https://github.com/bulletphysics/bullet3.git
-[submodule "ext/CEGUI/stb"]
-	path = ext/CEGUI/stb
-	url = https://github.com/nothings/stb.git
 [submodule "3rdparty/portable-file-dialogs"]
 	path = 3rdparty/portable-file-dialogs
 	url = https://github.com/samhocevar/portable-file-dialogs.git