Remove extraneous files; update themes submodule

- A preconfigured config.ini is probably not useful these days. There's
  an options menu now.
- Removed gs4 background - already in vanilla pack.
- Themes submodule should probably be configured to track master. Meh.
  Submodules can be so finicky. It probably doesn't belong in this repo
  anyway - it should be in the vanilla repo (currently invisible).

Author: oldmud0

.gitlab-ci.yml

@@ -123,7 +123,6 @@ build windows i686:
     mkdir base
     mkdir base/themes
     cp -a ../base/themes/default base/themes/
-    cp -a ../base/config.ini base/config.sample.ini
     cp -a ../base/serverlist.txt base/serverlist.sample.txt
 
 # Miscellaneous files