Make example binaries proper cargo examples (#7019)

Should not be built by default with `cargo build`
Instead are compiled with `cargo test` to avoid bitrot
Run with `cargo run -p ... --example ...`

crates/nu-system/Cargo.toml

@@ -9,10 +9,6 @@ license = "MIT"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
-[[bin]]
-name = "ps"
-path = "src/main.rs"
-
 [dependencies]
 libc = "0.2"
 log = "0.4"