Rollup merge of #84903 - hyd-dev:dead-check-in-alloc-msg, r=RalfJung
Rollup merge of #84903 - hyd-dev:dead-check-in-alloc-msg, r=RalfJung

Remove `rustc_middle::mir::interpret::CheckInAllocMsg::NullPointerTest`

Removing it per https://github.com/rust-lang/rust/pull/84842#discussion_r625589674: it's a dead enum variant.

Note that `PointerArithmeticTest` also seems dead:
```
$ rg -F PointerArithmeticTest -C5
compiler/rustc_middle/src/mir/interpret/error.rs
169-
170-/// Details of why a pointer had to be in-bounds.
171-#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
172-pub enum CheckInAllocMsg {
173-    MemoryAccessTest,
174:    PointerArithmeticTest,
175-    InboundsTest,
176-}
177-
178-impl fmt::Display for CheckInAllocMsg {
179-    /// When this is printed as an error the context looks like this
--
182-        write!(
183-            f,
184-            "{}",
185-            match *self {
186-                CheckInAllocMsg::MemoryAccessTest => "memory access",
187:                CheckInAllocMsg::PointerArithmeticTest => "pointer arithmetic",
188-                CheckInAllocMsg::InboundsTest => "inbounds test",
189-            }
190-        )
191-    }
192-}
```
Not sure if that is also desirable to be removed, however.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust