lang: remove problematic offsetof macro in favor of built-in offset_of

With rust 1.93 the `deref_nullptr` lint changed from a warning to an
error [0].
Instead of fixing this macro to not use nullptr anymore, we can just
remove it completely, as nowadays one can use the built-in offset_of
macro [1], which is available since rust 1.77.

Further, checking the source of all our crates [3] did not show any
current usage anymore.

[0]: rust-lang/rust#148122
[1]: https://doc.rust-lang.org/stable/std/mem/macro.offset_of.html
[2]: grep -R 'offsetof!' /usr/share/cargo/registry

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Acked-by: Christian Ebner <c.ebner@proxmox.com>
Link: https://lore.proxmox.com/20260126141215.401447-1-m.sandoval@proxmox.com
 [TL: add references and background for why it's fine to remove]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

Author: maximiliano-s

proxmox-lang/src/lib.rs

@@ -66,28 +66,6 @@ macro_rules! static_assert_size {
     };
 }
 
-/// Evaluates to the offset (in bytes) of a given member within a struct
-///
-/// ```
-/// # use proxmox_lang::offsetof;
-///
-/// #[repr(C)]
-/// struct Stuff {
-///     first: u32,
-///     second: u32,
-/// }
-///
-/// assert_eq!(offsetof!(Stuff, second), 4);
-///
-/// ```
-#[deprecated = "use std::mem::offset_of! instead"]
-#[macro_export]
-macro_rules! offsetof {
-    ($ty:ty, $field:ident) => {
-        unsafe { &(*(std::ptr::null::<$ty>())).$field as *const _ as usize }
-    };
-}
-
 /// Shortcut for generating an `&'static CStr`.
 ///
 /// This takes a *string* (*not* a *byte-string*), appends a terminating zero, and calls