rust: remove unneeded Rust 1.87.0 `allow(clippy::ptr_eq)`

For the Rust 1.87.0 release, Clippy was expected to warn with:

    error: use `core::ptr::eq` when comparing raw pointers
       --> rust/kernel/list.rs:438:12
        |
    438 |         if self.first == item {
        |            ^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::eq(self.first, item)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_eq
        = note: `-D clippy::ptr-eq` implied by `-D warnings`
        = help: to override `-D warnings` add `#[allow(clippy::ptr_eq)]`

However, a backport to relax a bit the `clippy::ptr_eq` finally landed,
and thus Clippy did not warn by the time the release happened.

Thus remove the `allow`s added back then, which were added just in case
the backport did not land in time.

See commit a39f3087 ("rust: allow Rust 1.87.0's `clippy::ptr_eq`
lint") for details.

Link: https://github.com/rust-lang/rust/pull/140859

 [1]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250520182125.806758-1-ojeda@kernel.org


[ Reworded for clarity. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>