remove unused imports: Deserialize, Serialize compiler warning for nu-protocol/src/example.rs (#8514)

when running the tests inside nu-protocol we were getting a compiler
warning...
this PR removes the compiler warning from nu-protocol.
by adding
```rust
#[allow(unused_imports)]
```

crates/nu-protocol/src/example.rs

@@ -1,4 +1,5 @@
 use crate::Value;
+#[allow(unused_imports)]
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug)]