[css-anchor-position] Remove lingering extra definition of the position fallback origin

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1507,10 +1507,6 @@ 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=].
-
 <div algorithm>
 	To <dfn>apply a position option</dfn> to an element |el|,
 	given a [=position option=] |new styles|: