rust: kernel: remove support for unused host `#[test]`s

Since commit 028df914 ("rust: str: convert `rusttest` tests into
KUnit"), we do not have anymore host `#[test]`s that run in the host.

Moreover, we do not plan to add any new ones -- tests should generally
run within KUnit, since there they are built the same way the kernel
does. While we may want to have some way to define tests that can also
be run outside the kernel, we still want to test within the kernel too
[1], and thus would likely use a custom syntax anyway to define them.

Thus simplify the `rusttest` target by removing support for host
`#[test]`s for the `kernel` crate.

This still maintains the support for the `macros` crate, even though we
do not have any such tests there.

Link: https://lore.kernel.org/rust-for-linux/CABVgOS=AKHSfifp0S68K3jgNZAkALBr=7iFb=niryG5WDxjSrg@mail.gmail.com/

 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20250726180750.2735836-1-ojeda@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>