Remove GC.md

We should removing GC.md as part of #1066. It's more aspirational that concrete, and we're now tracking GC work with #1079 as well as in https://github.com/WebAssembly/gc/.

Caveat before removing this:

* Other pages link to it. I'll clean up.
* Some of the content may still be valid. We should move it to the GC proposal instead so it doesn't become stale.