更新 book.json，代码色彩高亮换为默认的 highlight，并去除已经不再使用的 autocover 等

Signed-off-by: Tao Wang <twang2218@gmail.com>

book.json

@@ -2,30 +2,14 @@
   "title": "Docker -- 从入门到实践",
   "author": "yeasy",
   "plugins": [
-        "-autocover",
     "image-captions",
-        "github-buttons",
-        "latex-codecogs",
-        "prism", 
-        "-highlight"
+    "github-buttons"
   ],
   "pluginsConfig": {
-        "autocover": {
-            "font": {
-                "size": null,
-                "family": "Impact",
-                "color": "#000"
-            },
-            "size": {
-                "w": 1800,
-                "h": 2360
-            },
-            "background": {
-                "color": "#FFF"
-            }
-        },
     "image-captions": {
-            "attributes": { "width": "600" },
+      "attributes": {
+        "width": "600"
+      },
       "caption": "图 _PAGE_LEVEL_._PAGE_IMAGE_NUMBER_ - _CAPTION_"
     },
     "github-buttons": {