rust: remove spurious use core::fmt::Debug

We want folks to use `kernel::fmt` but this is only used for `derive` so
can be removed entirely.

This backslid in commit ea60cea ("rust: add `Alignment` type").

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20251018-cstr-core-v18-9-9378a54385f8@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: tamird

rust/kernel/ptr.rs

@@ -2,7 +2,6 @@
 
 //! Types and functions to work with pointers and addresses.
 
-use core::fmt::Debug;
 use core::mem::align_of;
 use core::num::NonZero;