Removed image_id property from books & shelves api docs

This was either not provided or not provided for the last 18 months.
Likely not providing much value as-is so removing.

Closes #3474

dev/api/responses/books-list.json

@@ -9,8 +9,7 @@
       "updated_at": "2019-12-11T20:57:31.000000Z",
       "created_by": 1,
       "updated_by": 1,
-      "owned_by": 1,
-      "image_id": 3
+      "owned_by": 1
     },
     {
       "id": 2,
@@ -21,8 +20,7 @@
       "updated_at": "2019-12-11T20:57:23.000000Z",
       "created_by": 4,
       "updated_by": 3,
-      "owned_by": 3,
-      "image_id": 34
+      "owned_by": 3
     }
   ],
   "total": 14

dev/api/responses/books-update.json

@@ -7,6 +7,5 @@
   "updated_at": "2020-01-12T14:16:10.000000Z",
   "created_by": 1,
   "updated_by": 1,
-  "owned_by": 1,
-  "image_id": 452
+  "owned_by": 1
 }

dev/api/responses/shelves-list.json

@@ -9,8 +9,7 @@
       "updated_at": "2020-04-10T13:00:45.000000Z",
       "created_by": 4,
       "updated_by": 1,
-      "owned_by": 1,
-      "image_id": 31
+      "owned_by": 1
     },
     {
       "id": 9,
@@ -21,8 +20,7 @@
       "updated_at": "2020-04-10T13:00:58.000000Z",
       "created_by": 4,
       "updated_by": 1,
-      "owned_by": 1,
-      "image_id": 28
+      "owned_by": 1
     },
     {
       "id": 10,
@@ -33,8 +31,7 @@
       "updated_at": "2020-04-10T13:00:53.000000Z",
       "created_by": 4,
       "updated_by": 1,
-      "owned_by": 4,
-      "image_id": 30
+      "owned_by": 4
     }
   ],
   "total": 3

dev/api/responses/shelves-update.json

@@ -6,7 +6,6 @@
   "created_by": 1,
   "updated_by": 1,
   "owned_by": 1,
-  "image_id": 501,
   "created_at": "2020-04-10T13:24:09.000000Z",
   "updated_at": "2020-04-10T13:48:22.000000Z"
 }