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

docs/_static/css/custom.css

 .strike {
 	text-decoration: line-through;
 }
-
-/*
-	This fixes the vertical position of list markers when the first
-	element in the <li> is a <pre> block
-
-	Scrolling inside the <pre> block is still working as expected
-*/
-.rst-content pre.literal-block,
-.rst-content div[class^='highlight'] pre {
-	overflow: visible;
-}