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.
2c7bf41b97