Remove the `precise_pointer_size_matching` feature gate
`usize` and `isize` are special for pattern matching because their range might depend on the platform. To make code portable across platforms, the following is never considered exhaustive:
```rust
let x: usize = ...;
match x {
0..=18446744073709551615 => {}
}
```
Because of how rust handles constants, this also unfortunately counts `0..=usize::MAX` as non-exhaustive. The [`precise_pointer_size_matching`](https://github.com/rust-lang/rust/issues/56354) feature gate was introduced both for this convenience and for the possibility that the lang team could decide to allow the above.
Since then, [half-open range patterns](https://github.com/rust-lang/rust/issues/67264) have been implemented, and since #116692 they correctly support `usize`/`isize`:
```rust
match 0usize { // exhaustive!
0..5 => {}
5.. => {}
}
```
I believe this subsumes all the use cases of the feature gate. Moreover no attempt has been made to stabilize it in the 5 years of its existence. I therefore propose we retire this feature gate.
Closes https://github.com/rust-lang/rust/issues/56354
81b6263dd0