[lex.ccon, lex.string] Remove unnecessary linebreaks and use bnf instead of bnftab.

Tab alignment is no longer required after everything has been made
to fit on a single line.

Author: tkoeppe

source/lex.tex

@@ -1064,13 +1064,12 @@
     c-char-sequence c-char
 \end{bnf}
 
-\begin{bnftab}
+\begin{bnf}
 \nontermdef{c-char}\br
-\>\textnormal{any member of the source character set except}\br
-\>\>\textnormal{the single-quote \terminal{'}, backslash \terminal{\textbackslash}, or new-line character}\br
-\>escape-sequence\br
-\>universal-character-name
-\end{bnftab}
+    \textnormal{any member of the source character set except the single-quote \terminal{'}, backslash \terminal{\textbackslash}, or new-line character}\br
+    escape-sequence\br
+    universal-character-name
+\end{bnf}
 
 \begin{bnf}
 \nontermdef{escape-sequence}\br
@@ -1407,13 +1406,12 @@
     s-char-sequence s-char
 \end{bnf}
 
-\begin{bnftab}
+\begin{bnf}
 \nontermdef{s-char}\br
-\>\textnormal{any member of the source character set except}\br
-\>\>\textnormal{the double-quote \terminal{"}, backslash \terminal{\textbackslash}, or new-line character}\br
-\>escape-sequence\br
-\>universal-character-name
-\end{bnftab}
+    \textnormal{any member of the source character set except the double-quote \terminal{"}, backslash \terminal{\textbackslash}, or new-line character}\br
+    escape-sequence\br
+    universal-character-name
+\end{bnf}
 
 \begin{bnf}
 \nontermdef{raw-string}\br