Track theme submodule via SSH URL, drop CI-only override

origin is already an SSH remote for this repo, so there's no real
reason for the submodule to stay on HTTPS. Removes the runtime
'git config submodule...url' override from the workflow, which is
now redundant.

.gitea/workflows/build-deploy.yml

@@ -38,10 +38,6 @@ jobs:
 
       - name: Fetch theme submodule
         run: |
-          # Override the submodule's tracked HTTPS URL for this checkout only,
-          # so it's fetched over SSH with the deploy key instead of needing a
-          # token with access to both repos.
-          git config submodule.themes/latex.url git@git.haemka.in:hmk/pelican-latex.git
           GIT_SSH_COMMAND="ssh -4 -i ~/.ssh/deploy_key -o StrictHostKeyChecking=yes" \
             git submodule update --init --recursive