Clear WASM warnings: migrate RequestInit setters, drop dead structs
The WASM build was emitting 7 warnings:
- Five deprecation warnings on `web_sys::RequestInit::{method,
headers, body}`. web-sys moved to setter-style API
(`set_method`, `set_headers`, `set_body`) in a recent release.
Migrated `wasm_transport.rs` accordingly; the new API takes `&self`
so `let mut opts = RequestInit::new()` becomes `let opts = ...`.
- Two dead-code warnings on `EmptyResult` and `UriResult` DTOs in
`wasm_util.rs`. Neither has any callers after the last few rounds
of WASM surface cleanup — deleted.
Post-change the WASM build is warning-clean:
cargo build -p opake-wasm --target wasm32-unknown-unknown
→ Finished, 0 warnings