How do I get this smoothed-over bolt out of my toilet?

I'm replacing the guts of my toilet, and I have tk take the whole tank off to do so. This particular bolt, which i believe is a large flathead, appears to have totally rusted (or something) over and is now just a round, smooth dome. Rest assured, there is no where to get a flathead screwdriver in there. I tried scraping at the slit with a knife, but it barely scratched it.

How do I get this thing out?!