Remove unused Singleton from codec.cc (#1499)

dictionary/system/codec.cc no longer uses Singleton. Let's remove
unnecessary include and dependency.

PiperOrigin-RevId: 913011107

Author: yukawa

src/dictionary/system/BUILD.bazel

@@ -46,7 +46,6 @@ mozc_cc_library(
     ],
     deps = [
         ":words_info",
-        "//base:singleton",
         "//base:util",
         "//base:vlog",
         "//dictionary:dictionary_token",

src/dictionary/system/codec.cc

@@ -39,7 +39,6 @@
 #include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/span.h"
-#include "base/singleton.h"
 #include "base/util.h"
 #include "base/vlog.h"
 #include "dictionary/dictionary_token.h"