rust: workqueue: remove unneeded ``#[allow(clippy::new_ret_no_self)]`

rust: workqueue: remove unneeded ``#[allow(clippy::new_ret_no_self)]`

Perform the same clean commit b2516f7af9d2 ("rust: kernel: remove
`#[allow(clippy::new_ret_no_self)]`") did for a case that appeared in
workqueue in parallel in commit 7324b88975c5 ("rust: workqueue: add
helper for defining work_struct fields"):

    Clippy triggered a false positive on its `new_ret_no_self` lint
    when using the `pin_init!` macro. Since Rust 1.67.0, that does
    not happen anymore, since Clippy learnt to not warn about
    `-> impl Trait<Self>` [1][2].

    The kernel nowadays uses Rust 1.72.1, thus remove the `#[allow]`.

Link: https://github.com/rust-lang/rust-clippy/issues/7344
Link: https://github.com/rust-lang/rust-clippy/pull/9733
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>