rust: device: Remove explicit import of CStrExt

Remove the explicit import of CStrExt. When CONFIG_PRINTK is disabled
this import causes a build error:

error: unused import: `crate::str::CStrExt`
--> rust/kernel/device.rs:17:5
|
17 | use crate::str::CStrExt as _;
| ^^^^^^^^^^^^^^^^^^^
|
= note: `-D unused-imports` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(unused_imports)]`

error: aborting due to 1 previous error

CStrExt is covered by prelude::* so the explicit import is redundant.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Fixes: 3b83f5d5e78a ("rust: replace `CStr` with `core::ffi::CStr`")
Link: https://patch.msgid.link/20260106000320.2593800-1-fujita.tomonori@gmail.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>