Remove `no_std` and `feature(...)` crate attributes

Since PR https://github.com/Rust-for-Linux/linux/pull/635 ("rust:
avoid the need of crate attributes in kernel modules") in the main
repository, these attributes are handled by the build system, thus
there is no need (and it is an error) to specify them.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust_out_of_tree.rs

@@ -2,9 +2,6 @@
 
 //! Rust out-of-tree sample
 
-#![no_std]
-#![feature(allocator_api, global_asm)]
-
 use kernel::prelude::*;
 
 module! {