Remove now-redundant case in the definition of flatten

Author: eernstg

specification/dartLangSpec.tex

@@ -11775,9 +11775,6 @@ \subsection{Function Expressions}
 as follows, using the first applicable case:
 
 \begin{itemize}
-\item If $T$ is \code{$S$?}\ for some $S$
-  then \DefEquals{\flatten{T}}{\code{\flatten{S}?}}.
-
 \item If $T$ is \code{$X$\,\&\,$S$}
   for some type variable $X$ and type $S$ then
   \begin{itemize}