Use atom-feed submodule from theme

Author: jarrodmillman

.gitmodules

@@ -1,6 +1,3 @@
 [submodule "themes/scientific-python-hugo-theme"]
 	path = themes/scientific-python-hugo-theme
 	url = https://github.com/scientific-python/scientific-python-hugo-theme
-[submodule "themes/hugo-atom-feed"]
-	path = themes/hugo-atom-feed
-	url = https://github.com/stefanv/hugo-atom-feed