[class] Replace "ncbnftab" environment with "ncsimplebnf" where no actual tabbing is used.

Author: tkoeppe

source/classes.tex

@@ -1309,9 +1309,9 @@
 \pnum
 A \grammarterm{member-declarator} of the form
 
-\begin{ncbnftab}
+\begin{ncsimplebnf}
 \opt{identifier} \opt{attribute-specifier-seq} \terminal{:} constant-expression \opt{brace-or-equal-initializer}
-\end{ncbnftab}
+\end{ncsimplebnf}
 
 \indextext{\idxcode{:}!bit-field declaration}%
 \indextext{declaration!bit-field}%
@@ -1670,9 +1670,9 @@
 \pnum
 A union of the form
 
-\begin{ncbnftab}
+\begin{ncsimplebnf}
 \terminal{union} \terminal{\{} member-specification \terminal{\}} \terminal{;}
-\end{ncbnftab}
+\end{ncsimplebnf}
 
 is called an \defn{anonymous union}; it defines an unnamed type and
 an unnamed object of that type called an \defn{anonymous union object}.