docs: remove remaining CSS workaround (#2737)

Remove a workaround for a CSS issue in older Firefox versions, which was
recently fixed (at some point between Firefox 102 and 108). Firefox 108+
now displays the bullet points at the right vertical position even without
the hack in [1].

[1] https://gluon.readthedocs.io/en/latest/releases/v2018.1.html#site-conf