I ran Brent Ozar's sp_blitz on one of my production server. IT tagged a few tables in the MSDB database as being user created. They all follow the pattern dbo.DTA_*. Do these table come with MSDB, or were they added by something/someone?
These are likely left over from Microsoft's Database Tuning Advisor. You should be able to get information about the sessions with this stored procedure:
After identifying the sessions that you do not need you can remove them with this stored procedure supplying the session id using one as an example: