Remove special guidance around wasm

Go is going to support a platform like `wasm/wasip1`, `wasm/wasip2`, and eventually `wasm/wasi`. We should remove the guidance that Wasm is a special case, and fallback to following Go's guidance when describing a platform.

Author: imjasonh

image-index.md

@@ -58,7 +58,6 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 
         This REQUIRED property specifies the operating system.
         Image indexes SHOULD use, and implementations SHOULD understand, values listed in the Go Language document for [`GOOS`][go-environment2].
-        An exception to this guidance is provided for Wasm applications that rely on the WASI system interface, where the `os` SHOULD be `wasi`.
 
     - **`os.version`** *string*