MM-55710: Remove join with channel in GetFlaggedPosts (#25513)

https://mattermost.atlassian.net/browse/MM-55710

```release-note
NONE
```

server/channels/store/sqlstore/post_store.go

@@ -505,13 +505,10 @@ func (s *SqlPostStore) getFlaggedPosts(userId, channelId, teamId string, offset
 			WHERE
 				ChannelId IN (
 					SELECT
-						Id
+						ChannelId
 					FROM
-						Channels,
 						ChannelMembers
-					WHERE
+					WHERE UserId = ?
-						Id = ChannelId
-						AND UserId = ?
 				)
 				TEAM_FILTER
             ORDER BY CreateAt DESC