[expr.sub] Make deprecation of commas in brackets normative.

Core language deprecations are stated in core wording, and then
cross-referenced from Annex D. All current references to deprecating
this feature are notes, hence there is no normative deprecation.

This change promotes a note to non-note, normative text.

Author: AlisdairM

source/expressions.tex

@@ -2954,12 +2954,10 @@
 The expression \tcode{E1} is sequenced before the expression \tcode{E2}.
 
 \pnum
-\begin{note}
 A comma expression\iref{expr.comma}
 appearing as the \grammarterm{expr-or-braced-init-list}
 of a subscripting expression is deprecated;
 see \ref{depr.comma.subscript}.
-\end{note}
 
 \pnum
 \begin{note}