Remove z-index from .bs-content as it breaks ui elements ...
...such as OOJS-UI (on Special:RecentChanges i.e.) that usually are rendered with z-index 1 or auto. As far as i have seen, there was no side effect when removing this. It is unclear why it is set that high in the first place. I did not find any ticket or the original commit due to this beeing present since a long time through multiple refactoring
(cherry picked from commit 9b37a4aa6a7e6f2e250fc2ecb92f9be8cdfa56da)