MM-62900: Avoid redundant query for channelID while indexing file (#30289)

The file already has the ChannelID field. Therefore, we avoid querying
the database again. This sometimes causes errors in customer environments
where  there is noticeable replication lag, causing files not to be
indexed entirely.

https://mattermost.atlassian.net/browse/MM-62900

```release-note
NONE
```

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/searchlayer/file_info_layer.go

@@ -26,15 +26,6 @@ func (s SearchFileInfoStore) indexFile(rctx request.CTX, file *model.FileInfo) {
 					return
 				}
 				channelId := file.ChannelId
-				if file.PostId != "" {
-					post, postErr := s.rootStore.Post().GetSingle(rctx, file.PostId, false)
-					if postErr != nil {
-						rctx.Logger().Error("Couldn't get post for file for SearchEngine indexing.", mlog.String("post_id", file.PostId), mlog.String("search_engine", engineCopy.GetName()), mlog.String("file_info_id", file.Id), mlog.Err(postErr))
-						return
-					}
-					channelId = post.ChannelId
-				}
-
 				if channelId == "" {
 					rctx.Logger().Error("Couldn't associate file with a channel for file for SearchEngine indexing.", mlog.String("search_engine", engineCopy.GetName()), mlog.String("file_info_id", file.Id))
 					return