json_tokener: optimize check for number characters

speedup for 32-bit: ~5%
speedup for 64-bit: ~3%

json_object.c

@@ -41,7 +41,6 @@
 // Don't define this.  It's not thread-safe.
 /* #define REFCOUNT_DEBUG 1 */
 
-const char *json_number_chars = "0123456789.+-eE";
 const char *json_hex_chars = "0123456789abcdefABCDEF";
 
 static struct json_object* json_object_new(enum json_type o_type);