json_spirit: remove unused typedef

json_spirit: remove unused typedef

In file included from json_spirit/json_spirit_writer.cpp:7:0:
json_spirit/json_spirit_writer_template.h: In function 'String_type json_spirit::non_printable_to_string(unsigned int)':
json_spirit/json_spirit_writer_template.h:37:50: warning: typedef 'Char_type' locally defined but not used [-Wunused-local-typedefs]
         typedef typename String_type::value_type Char_type;

(Also, ha ha, this file uses \r\n.)

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 6abae35a3952e5b513895267711fea63ff3bad09)