Fix build errors: FTPS AsyncRead bridge, TLS connector type, unsafe env::set_var, dead code

Verified with cargo check + cargo run --example printer_polymorphism.

src/plate_changer/mod.rs

@@ -1,8 +1,6 @@
 mod gpio;
 mod serial;
 
-pub use gpio::SensorState;
-
 use thiserror::Error;
 use tracing::{info, warn};