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

Before a change to relax the lint was backported to Rust 1.87.0 before
its 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)]`

The backported finally landed indeed, thus remove the `allow`s we added
back then, which were added just in case the backport did not land
in time.

See commit a39f308 ("rust: allow Rust 1.87.0's `clippy::ptr_eq`
lint") for details.

Link: rust-lang/rust#140859 [1]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250520182125.806758-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/alloc/kvec.rs

@@ -2,9 +2,6 @@
 
 //! Implementation of [`Vec`].
 
-// May not be needed in Rust 1.87.0 (pending beta backport).
-#![allow(clippy::ptr_eq)]
-
 use super::{
     allocator::{KVmalloc, Kmalloc, Vmalloc},
     layout::ArrayLayout,

rust/kernel/list.rs

@@ -4,9 +4,6 @@
 
 //! A linked list implementation.
 
-// May not be needed in Rust 1.87.0 (pending beta backport).
-#![allow(clippy::ptr_eq)]
-
 use crate::sync::ArcBorrow;
 use crate::types::Opaque;
 use core::iter::{DoubleEndedIterator, FusedIterator};