Remove the dead ProjectHorizon submodule

Don't know what happened to this project, but its GitLab repository is again unavailable.
The Weekly snapshots are failing now that the CI can't check out all submodules.

Author: rdw-software

.gitmodules

@@ -130,9 +130,6 @@
 [submodule "Servers/horizon"]
 	path = Servers/horizon
 	url = https://github.com/rdw-forks/horizon.git
-[submodule "Servers/ProjectHorizon"]
-	path = Servers/ProjectHorizon
-	url = http://lab.projecthorizon.xyz/horizon/horizon.git
 [submodule "Tools/gr2-web"]
 	path = Tools/gr2-web
 	url = https://github.com/herenow/gr2-web.git