tabbed-panes: drop deadcode highlight style override

[chalin]

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?