Broken (or private) links in known-limitations.md

[vi]

They go to https://github.com/GaloisInc/C2Rust/issues/, but the issues are not viewable.

[glguy]

I've removed the broken links. It'd be nice to make all the internal issues external but we'll need to figure that out separately.

[thedataking]

I've removed the links from docs/known-limitations.md too. Closing.