If you remove those projects: fine, that's your decision.
If you don't remove them: make sure you mark them in the sense that you add a visible note saying "THIS PROJECT IS NO LONGER SUPPORTED", so that a visitor who stumbles upon your code knows that he's spending time on a dead project when he decides to use your code (he can have very good reasons to do so, for instance if he wants to revive the project).
Not removing your code can make sense if you ever intend to revisit your project. You never know what happens with your own archives (crashed disk drives, DVDs that are no longer legible,...) Keeping the code where it is now, can make sense.
Not removing your code can result in you being contacted about that code. For instance: I once made the mistake to add my phone number on a tutorial page (which was very stupid of me). Years after the project was discontinued, people still called me for support. If you live in Europe and people call you from the US during their business hours, this is a pain. Changing your phone number is the only option you have in that case. The same goes for mail addresses: make sure that you remove your priority mail address because people are going to contact you when they think they need your code, when they've tried it and when it doesn't work for them.
Finally: the internet never forgets. Even if you remove your project, there is a chance that other copies of your code are still out there somewhere. Again: I once left my phone number on a tutorial page, and even after I removed that page, people still called me because the page was copied on another site.