rust: kernel: remove usage of allocator_api unstable feature

With the adoption of `BoxExt` and `VecExt`, we don't need the functions
provided by this feature (namely the methods prefixed with `try_` and
different allocator per collection instance).

We do need `AllocError`, but we define our own as it is a trivial empty
struct.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20240328013603.206764-11-wedsonaf@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc/vec_ext.rs

@@ -2,8 +2,8 @@
 
 //! Extensions to [`Vec`] for fallible allocations.
 
-use super::Flags;
-use alloc::{alloc::AllocError, vec::Vec};
+use super::{AllocError, Flags};
+use alloc::vec::Vec;
 use core::result::Result;
 
 /// Extensions to [`Vec`].