Revert "Remove git submodules before git checkout (#1611)" (#1799)

This reverts commit ff07728fa3.

Removing the .git file from the git submodule is problematic when
updating buildroot: Files deleted stay present in the buildroot
directory (since their origin is no longer known).

The workaround has been introduced to allow building non-git submodule
releases (rel-6) on the same runners. Since rel-7 uses git submodule and
we stay with git submodule for the forseeable future, remove this work
around.

.github/workflows/dev.yml

@@ -77,10 +77,6 @@ jobs:
               return { "ref": context.ref }
             return { "ref": context.payload.pull_request.head.sha }
 
-      # Make sure directories of potentially removed submodules are cleaned correctly
-      - name: Cleanup git submodules
-        run: find . -name .git -type f -exec rm {} \;
-
       - name: Checkout source
         uses: actions/checkout@v2
         with: