cleanup .gitmodules

Author: pkestene

.gitmodules

@@ -9,16 +9,3 @@
 [submodule "external/backward-cpp"]
 	path = external/backward-cpp
 	url = https://github.com/bombela/backward-cpp.git
-[submodule "external/SphinxExtensions/breathelink"]
-	path = external/SphinxExtensions/breathelink
-	url = https://github.com/thewtex/breathelink.git
-[submodule "external/SphinxExtensions/breathe"]
-	path = external/SphinxExtensions/breathe
-	url = https://github.com/michaeljones/breathe.git
-	branch = v4.10.0
-[submodule "external/SphinxExtensions/gitstats"]
-	path = external/SphinxExtensions/gitstats
-	url = https://github.com/arnaudgelas/gitstats.git
-[submodule "external/SphinxExtensions/sphinx-bootstrap-theme"]
-	path = external/SphinxExtensions/sphinx-bootstrap-theme
-	url = https://github.com/ryan-roemer/sphinx-bootstrap-theme.git