Update DESIGN_DOC.md

doc/DESIGN_DOC.md

@@ -97,8 +97,6 @@ Paddle Serving的核心执行引擎是一个有向无环图，图中的每个节
 <p align="center">
     <br>
 <img src='user_groups.png' width = "320" height = "470">
-<img src='blank.png' width = "50" height = "470">
-<img src='coding_mode.png' width = "260" height = "470">
     <br>
 <p>