tabbed-panes: drop deadcode highlight style override

_Originally posted by @chalin in https://github.com/google/docsy/pull/1163#discussion_r945529251_

> As I understand it, a code-block cannot appear in a `tab-content` outside of a `tab-pane`, but there is an override of `highlight` in `tab-pane` already, so I think that this style change is dead code and should be removed. WDYT @deining @LisaFC?
