If two profiles were merged, then only 1 will remain on your watchlist. Here's how to make sure you really want to remove the profile, and then how to do it:
First, click the link on your watchlist to open the profile page. Look carefully at the profile to make sure it is one you want to no longer manage (or be on the trusted list for).
At the top of the profile, there are tabs. Click the Privacy tab and scroll down to where the managers and members of the trusted list are shown. For each person listed there, you will see a link to Remove as Manager. You can click that to remove yourself as manager, but you will still be on the trusted list and the profile will still show up on your watchlist. If you want to remove yourself completely, there is a button below the list that you can click to do that. If you do that then the profile will no longer be on your watchlist.