[expr.prim.id.unqual] Remove unused meta-variable (#8574)

Author: lprv

source/expressions.tex

@@ -1604,7 +1604,7 @@
 Otherwise,
 if the \grammarterm{unqualified-id}
 names a result binding\iref{dcl.contract.res}
-attached to a function \placeholder{f}
+attached to a function
 with return type \tcode{U},
 \begin{itemize}
 \item