Remove author information from "post" in merge_requests API.

From both experimenting with the API and reading the source code, this appears to not be used at all.

My guess is this was copypasta from somewhere else.

doc/api/merge_requests.md

@@ -336,14 +336,6 @@ Parameters:
 
 ```json
 {
-  "author": {
-    "id": 1,
-    "username": "admin",
-    "email": "admin@example.com",
-    "name": "Administrator",
-    "blocked": false,
-    "created_at": "2012-04-29T08:46:00Z"
-  },
   "note": "text1"
 }
 ```