Simplify JSON unicode serialization

It’s just an array of codepoints; no need to turn them into objects

src/hb-buffer-serialize.cc

@@ -214,13 +214,8 @@ _hb_buffer_serialize_unicode_json (hb_buffer_t *buffer,
     if (i)
       *p++ = ',';
 
-    *p++ = '{';
-
-    APPEND ("\"u\":");
     p += hb_max (0, snprintf (p, ARRAY_LENGTH (b) - (p - b), "%u", info[i].codepoint));
 
-    *p++ = '}';
-
     unsigned int l = p - b;
     if (buf_size > l)
     {