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: 48fadf440075 ("docs: Move rustdoc output, cross-reference it")
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>