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: 48fadf440075 ("docs: Move rustdoc output, cross-reference it")
Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Reviewed-by: Andreas Hindborg <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

0 files changed, 0 insertions, 0 deletions