Remove the ptr_unique lang item

Miri no longer uses it.

compiler/rustc_hir/src/lang_items.rs

@@ -345,9 +345,6 @@ language_item_table! {
     OwnedBox,                sym::owned_box,           owned_box,                  Target::Struct,         GenericRequirement::Minimum(1);
     GlobalAlloc,             sym::global_alloc_ty,     global_alloc_ty,            Target::Struct,         GenericRequirement::None;
 
-    // Experimental lang item for Miri
-    PtrUnique,               sym::ptr_unique,          ptr_unique,                 Target::Struct,         GenericRequirement::Exact(1);
-
     PhantomData,             sym::phantom_data,        phantom_data,               Target::Struct,         GenericRequirement::Exact(1);
 
     ManuallyDrop,            sym::manually_drop,       manually_drop,              Target::Struct,         GenericRequirement::None;

compiler/rustc_span/src/symbol.rs

@@ -1698,7 +1698,6 @@ symbols! {
         ptr_slice_from_raw_parts_mut,
         ptr_swap,
         ptr_swap_nonoverlapping,
-        ptr_unique,
         ptr_write,
         ptr_write_bytes,
         ptr_write_unaligned,

library/core/src/ptr/unique.rs

@@ -32,8 +32,6 @@ use crate::ptr::NonNull;
 )]
 #[doc(hidden)]
 #[repr(transparent)]
-// Lang item used experimentally by Miri to define the semantics of `Unique`.
-#[lang = "ptr_unique"]
 pub struct Unique<T: PointeeSized> {
     pointer: NonNull<T>,
     // NOTE: this marker has no consequences for variance, but is necessary