reader: get rid of a useless function

Useless since 58d7a1a1c7 (2006).

* src/parse-gram.y, src/reader.h (token_name): Remove, unused.

src/reader.h

@@ -38,7 +38,6 @@ typedef struct merger_list
 /* From the parser.  */
 extern int gram_debug;
 int gram_parse (void);
-char const *token_name (int type);
 
 
 /* From reader.c. */