Remove bullet3 submodule - appears that they history is messy, .gitmodules after shallow clone is filled with additional 100 MB where the src directory is about 7 MB

Author: AnastaZIuk

.gitmodules

@@ -24,9 +24,6 @@
 [submodule "3rdparty/libjpeg-turbo"]
 	path = 3rdparty/libjpeg-turbo
 	url = git@github.com:Devsh-Graphics-Programming/libjpeg-turbo.git
-[submodule "3rdparty/bullet3"]
-	path = 3rdparty/bullet3
-	url = git@github.com:Devsh-Graphics-Programming/bullet3.git
 [submodule "3rdparty/portable-file-dialogs"]
 	path = 3rdparty/portable-file-dialogs
 	url = git@github.com:Devsh-Graphics-Programming/portable-file-dialogs.git