rust: drm: gem: Drop Object::SIZE

Drive-by fix, it doesn't seem like anything actually uses this constant
anymore.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250908185239.135849-4-lyude@redhat.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Author: Lyude

rust/kernel/drm/gem/mod.rs

@@ -12,7 +12,7 @@ use crate::{
     prelude::*,
     types::{ARef, AlwaysRefCounted, Opaque},
 };
-use core::{mem, ops::Deref, ptr::NonNull};
+use core::{ops::Deref, ptr::NonNull};
 
 /// A type alias for retrieving a [`Driver`]s [`DriverFile`] implementation from its
 /// [`DriverObject`] implementation.
@@ -197,9 +197,6 @@ pub struct Object<T: DriverObject + Send + Sync> {
 }
 
 impl<T: DriverObject> Object<T> {
-    /// The size of this object's structure.
-    pub const SIZE: usize = mem::size_of::<Self>();
-
     const OBJECT_FUNCS: bindings::drm_gem_object_funcs = bindings::drm_gem_object_funcs {
         free: Some(Self::free_callback),
         open: Some(open_callback::<T>),