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.
dde7a7b809