I just re-read your last post and understand now, the maintenance plan is still looking for the reportserver db even though its gone.

From what I've read elsewhere, and since you've tried modifying the maintenance plan and still have the issue, you'll need to recreate the master maintenance plan so it can detect the database has been removed.

This is one of the reasons to use a custom backup script instead of maintenance plans. You can get started with Ola Hallagrens backup scripts here: http://ola.hallengren.com/sql-server-backup.html