rust: remove leftover mentions of the `alloc` crate

In commit 392e34b6bc22 ("kbuild: rust: remove the `alloc` crate and
`GlobalAlloc`") we stopped using the upstream `alloc` crate.

Thus remove a few leftover mentions treewide.

Cc: [e-mail] # Also to 6.12.y after the `alloc` backport lands
Fixes: 392e34b6bc22 ("kbuild: rust: remove the `alloc` crate and `GlobalAlloc`")
Reviewed-by: Danilo Krummrich <[e-mail]>
Reviewed-by: Andreas Hindborg <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Miguel Ojeda <[e-mail]>

Documentation/rust/quick-start.rst

···
 ****************************
 
 The Rust standard library source is required because the build system will
-cross-compile ``core`` and ``alloc``.
+cross-compile ``core``.
 
 If ``rustup`` is being used, run::
 

rust/kernel/lib.rs

···
 //! usage by Rust code in the kernel and is shared by all of them.
 //!
 //! In other words, all the rest of the Rust code in the kernel (e.g. kernel
-//! modules written in Rust) depends on [`core`], [`alloc`] and this crate.
+//! modules written in Rust) depends on [`core`] and this crate.
 //!
 //! If you need a kernel C API that is not ported or wrapped yet here, then
 //! do so first instead of bypassing this crate.

scripts/rustdoc_test_gen.rs

···
 //!   - Test code should be able to define functions and call them, without having to carry
 //!     the context.
 //!
-//!   - Later on, we may want to be able to test non-kernel code (e.g. `core`, `alloc` or
-//!     third-party crates) which likely use the standard library `assert*!` macros.
+//!   - Later on, we may want to be able to test non-kernel code (e.g. `core` or third-party
+//!     crates) which likely use the standard library `assert*!` macros.
 //!
 //! For this reason, instead of the passed context, `kunit_get_current_test()` is used instead
 //! (i.e. `current->kunit_test`).