Delete the wasmtime-wasi-c crate (#844)

This commit deletes the old C implementation of the original
`wasi_unstable` module, instead only leaving around our single
`wasmtime-wasi` crate as the implementation for both
`wasi_snapshot_preview1` and `wasi_unstable`.

This hasn't been discussed (AFAIK) up until now, so this is also a
proposal! Some thoughts in favor of this deletion I would have are:

* This has been off-by-default for ages
* We don't build or test any of this on CI
* Published binaries with `wasmtime` do not have this possibility
  enabled
* Future refactorings to the `wasmtime-wasi` crate will either need to
  work around how the C implementation is different or bring it up to
  speed.

This is motivated by the last bullet point where I was working on
getting `wasmtime-wasi` working purely as an implementation detail on
top of the `wasmtime` crate itself, but quickly ran into a case where
the CLI would need to multiplex all sorts of wasi implementations. In
any case I'm curious what others think, is this too soon? Is there
something remaining blocking this? (etc)

Cargo.lock

@@ -2078,7 +2078,6 @@ dependencies = [
  "wasmtime-obj",
  "wasmtime-runtime",
  "wasmtime-wasi",
- "wasmtime-wasi-c",
  "wasmtime-wast",
  "wat",
 ]
@@ -2277,24 +2276,6 @@ dependencies = [
  "wig",
 ]
 
-[[package]]
-name = "wasmtime-wasi-c"
-version = "0.9.0"
-dependencies = [
- "bindgen",
- "cmake",
- "cranelift-codegen",
- "cranelift-entity",
- "cranelift-wasm",
- "libc",
- "log",
- "more-asserts",
- "target-lexicon",
- "wasmtime-environ",
- "wasmtime-jit",
- "wasmtime-runtime",
-]
-
 [[package]]
 name = "wasmtime-wast"
 version = "0.9.0"