Remove unneded prefix argument from instantiate_wasi. (#584)

* Remove unneded prefix argument from `instantiate_wasi`.

This was an artifact of an earlier backwards-compatibility mechanism
which is no longer needed.

* Remove unneeded prefix arg from remaning uses

crates/test-programs/tests/wasm_tests/runtime.rs

@@ -53,7 +53,6 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
         Instance::from_handle(
             &store,
             wasmtime_wasi::instantiate_wasi_with_context(
-                "",
                 global_exports.clone(),
                 builder.build().context("failed to build wasi context")?,
             )