Remove host API implementations targeting WASIp2 prereleases.

Closes #65.

README.md

@@ -145,6 +145,10 @@ If your builtin requires multiple `.cpp` files, you can pass all of them to `add
 
 ### Providing a custom host API implementation
 
-The [host-apis](host-apis) directory contains implementations of the host API for different versions of WASI. Those can be selected by setting the `HOST_API` environment variable to the name of one of the directories. By default, the [wasi-0.2.0](host-apis/wasi-0.2.0) host API is used.
+The [host-apis](host-apis) directory can contain implementations of the host API for different 
+versions of WASI—or in theory any other host interface. Those can be selected by setting the 
+`HOST_API` environment variable to the 
+name of one of the directories. Currently, only an implementation in terms of [wasi-0.2.0]
+(host-apis/wasi-0.2.0) is provided, and used by default.
 
 To provide a custom host API implementation, you can set `HOST_API` to the (absolute) path of a directory containing that implementation.