[expr.const] Unmark introduction of "constant expression" as definition

Author: eisenwave

source/expressions.tex

@@ -8266,9 +8266,7 @@
 semantics depending on whether or not an expression satisfies these requirements.
 Expressions that satisfy these requirements,
 assuming that copy elision\iref{class.copy.elision} is not performed,
-are called
-\indexdefn{expression!constant}%
-\defnx{constant expressions}{constant expression}.
+are called constant expressions.
 \begin{note}
 Constant expressions can be evaluated
 during translation.