The Goblin wallet moved off the Nym mixnet to Tor: it now reaches relays
over a Tor circuit to their clearnet endpoint, so the relay-side
co-located mixnet exit is dead weight. Remove it entirely.
- delete the vendored `mixexit/` subcrate (drops the nym-sdk dependency
chain with it) and `src/exit.rs` (the in-process supervisor)
- un-wire the touch points: `pub mod exit;` (lib.rs), the `MixnetExit`
struct + `Settings.exit` field + its `Default` entry (config.rs), the
`exit::validate` / `exit::spawn` call sites (server.rs), the commented
`[exit]` block (config.toml)
- README: drop the "Mixnet exit" section and its feature bullet; add a
"Transport privacy (Tor)" note (wallets connect over Tor; an operator
can optionally front the relay with an onion at the deploy layer)
Pure subtraction: the exit was always optional and default-off, so no
behavior changes for anyone not already running it. Builds green;
cargo test and clippy pass.
2ro