json: don't call va_end() twice in json_build()

This was apparently left-over when json_buildv() was added, and
json_build() just became a wrapper for it.

src/shared/json.c

@@ -3108,8 +3108,6 @@ finish:
 
         free(stack);
 
-        va_end(ap);
-
         return r;
 }