Removed Casper-specific 4.0 code from CI

no issue

- the 4.0 Casper work is now in `main` and we don't need to switch
  branches any more

.github/workflows/test.yml

@@ -134,9 +134,6 @@ jobs:
       - run: npm install -g ghost-cli@latest
       - run: npm --no-git-tag-version version minor # We need to artificially bump the minor version to get migrations to run
 
-      - run: git checkout 4.0
-        working-directory: content/themes/casper
-
       - run: zip -r ghost.zip .
 
       - name: Clean Install
@@ -192,9 +189,6 @@ jobs:
       - run: yarn
       - run: grunt master
 
-      - run: git checkout 4.0
-        working-directory: content/themes/casper
-
       - run: echo "ghost_hash=$(git rev-parse --short HEAD)" >> $GITHUB_ENV
       - run: echo "ghost_admin_hash=$(git rev-parse --short HEAD)" >> $GITHUB_ENV
         working-directory: core/client