MM-11434 Performance improvements for post metadata (#9849)

* Remove unused error return value from PreparePostForClient

* Remove unused error return value from PreparePostListForClient

* MM-11434 Parallelize PreparePostListForClient

* MM-11434 Skip looking reactions and files on post whenever possible

* Add note about the use of deprecated fields

api4/channel.go

@@ -488,10 +488,7 @@ func getPinnedPosts(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	clientPostList, err := c.App.PreparePostListForClient(posts)
-	if err != nil {
-		mlog.Error("Failed to prepare posts for getFlaggedPostsForUser response", mlog.Any("err", err))
-	}
+	clientPostList := c.App.PreparePostListForClient(posts)
 
 	w.Header().Set(model.HEADER_ETAG_SERVER, clientPostList.Etag())
 	w.Write([]byte(clientPostList.ToJson()))