Remove obsolete comment about boldface lexical units

The comment was obsolete for two reasons:
- The boldface formatting was lost in the conversion to LaTeX.
- There are annotations making use of both S-CHAR, NON-DIGIT, and UNSIGNED-INTEGER, none of which were marked with boldface in 3.4.  With so many of the lexical units actually being used, there seems to be little value in marking them.

chapters/syntax.tex

@@ -16,9 +16,7 @@ \section{Lexical conventions}\label{lexical-conventions}
 \end{tabular}
 \end{center}
 
-The following lexical units are defined (the ones in boldface are the
-ones used in the grammar, the rest are just internal to the definition
-of other lexical units):
+The following lexical units are defined:
 \begin{lstlisting}[language=grammar]
 IDENT = NON-DIGIT { DIGIT | NON-DIGIT } | Q-IDENT
 Q-IDENT = "'" { Q-CHAR | S-ESCAPE } "'"