Clean up unneeded Debug derive macro import
Presumably this used to be a handwritten Debug impl which needed the
import, later replaced by #[derive(Debug)] which is present in the
prelude but masks the unused_imports warning.
diff --git a/src/exception.rs b/src/exception.rs
index f61e8fa..e0c2f1e 100644
--- a/src/exception.rs
+++ b/src/exception.rs
@@ -1,5 +1,5 @@
use alloc::boxed::Box;
-use core::fmt::{self, Debug, Display};
+use core::fmt::{self, Display};
/// Exception thrown from an `extern "C++"` function.
#[derive(Debug)]