What do I do about an unfixable F when I am applying for a graduate program?
Computer science professors don't care what grades you got in philosophy. Even more so because this was an advanced philosophy class, not an entry-level course. And even more so because this was 3+ years ago before you started computer science, so it doesn't show an inability to reason or communicate or manage your time. 

Better yet, you have a very clear narrative. When your application isn't perfect, it's nice to have a simple, clean, verifiable reason for the imperfection. In your case, it's very simple: you were terrible at philosophy, you're great at computer science. This is a great narrative, and the F only adds to it.

You should focus on computer science. Here's what's going to happen: the committee will look at your transcript and very quickly figure out that you were terrible at philosophy and good at computer science. Then they're going to want to know how good at computer science you are -- merely average? Or amazing? This is where your research, publications, letters of recommendation, advanced coursework, etc. comes in. Further, you should be very careful when writing your statement of purpose -- you'll need to address your previous major, but there's no need to make excuses for it, just say that it wasn't a good fit and then focus on your current work. 

Even if you could retake that class and get rid of the F, you shouldn't bother. It just doesn't gain you anything, and there are better uses for your time.