Remove Simple Hugo Theme

We are now using the Beautiful Hugo theme, remove the Simple Hugo Theme as
it's no longer required.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

.gitmodules

-[submodule "themes/simple-hugo-theme"]
-	path = themes/simple-hugo-theme
-	url = https://github.com/Xzya/simple-hugo-theme.git

simple-hugo-theme @ 470ab12e

-Subproject commit 470ab12e811ce71c424e43419b52e8356b8e0f61