[expr.const] Remove duplicate cross-reference (#4055)

Author: JohelEGP

source/expressions.tex

@@ -6928,7 +6928,7 @@
 \end{itemize}
 
 \item
-an lvalue-to-rvalue conversion\iref{conv.lval}
+an lvalue-to-rvalue conversion
 that is applied to a glvalue
 that refers to a non-active member of a union or a subobject thereof;