kbuild: remove old Rust docs output path

The Rust code documentation output path moved from `rust/doc` to
`Documentation/output/rust/rustdoc`. The `make cleandocs` target
takes care of cleaning it now since it is integrated with the rest
of the documentation.

Thus remove the old reference.

Fixes: 48fadf44

 ("docs: Move rustdoc output, cross-reference it")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20231018160145.1017340-2-ojeda@kernel.org


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