You are seeing it in your taskbar because it was one of the five most recently played games you own. Once a game has been uninstalled from your library, if you do not want to ever play it again, you need to disassociate it with your steam account.
You can do this by first navigating to
Help > Steam Support

You should now see a page that looks like this:

What we want to do is navigate down to the search box (highlighted in red) and type in the name of the game, in your case Torchlight II Demo
Once you have found Torchlight II Demo you will need to click on it.
You will see a list of options. (I am removing the stanley parable demo from my library for the sake of this demonstration)
The last option on the list should say
I want to permanently remove this game from my account

Once you select this and confirm it, the game will be disassociated with your steam account. The change may be immediate, but if not - try restarting steam.