kbuild: rust: remove now-unneeded `rusttest` custom sysroot handling

Since we dropped our custom `alloc` in commit 9d0441ba ("rust: alloc:
remove our fork of the `alloc` crate"), there is no need anymore to keep
the custom sysroot hack.

Thus delete it, which makes the target way simpler and faster too.

This also means we are not using Cargo for anything at the moment,
and that no download is required anymore, so update the main `Makefile`
and the documentation accordingly.

Link: https://lore.kernel.org/r/20240528163502.411600-1-ojeda@kernel.org


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