[css-anchor-position] Remove duplicate dfn

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1503,10 +1503,7 @@ These modified styles are applied to the element via [=interleaving=],
 so they affect [=computed values=]
 (and can trigger transitions/etc)
 even tho they depend on layout and [=used values=].
-
-They are applied as <dfn export>position fallback origin</dfn>,
-a new [=cascade origin=] positioned between the [=author origin=]
-and the [=animation origin=].
+They are applied as part of the [=position fallback origin=].
 
 <div algorithm>
 	To <dfn>apply a position option</dfn> to an element |el|,