Also remove all the wasi-libc files we used to ship, but never compile.

The latest wasi-libc HEAD has an extra commit (a6f871343313220b76009827ed0153586361c0d5), which makes preopen initialization lazy.

Unfortunately, that breaks quite a lot of things on our end. Applications now need to explicitly call __wasilibc_populate_preopens() everywhere when the libc is linked. That can wait after 0.11.