Remove `ProjectionElem::Subtype`

[ouz-a]

This PR reverts ProjectionElem::Subtype which was introduced in https://github.com/rust-lang/rust/pull/115025 which attempetd to fix https://github.com/rust-lang/rust/issues/107205 but couldn't completely solve the issue due to https://github.com/rust-lang/rust/issues/112651#issuecomment-1833491961, so leaving this as half-solution is more dangerous than completely removing it.

r? @lcnr

[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

This PR changes Stable MIR

cc @oli-obk, @celinval, @spastorino, @ouz-a

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

Some changes occurred in engine.rs, potentially modifying the public API of ObligationCtxt.

cc @lcnr, @compiler-errors

[ouz-a]

I think this should be it, removed everything, please let me know if there is anything else that catches your eye

[lcnr]

there's an ongoing convo in #112651 as explicit subtyping may work out after all :sweat_smile: going to wait before merging this for now. sorry @ouz-a xx

[bors]

:umbrella: The latest upstream changes (presumably #118692) made this pull request unmergeable. Please resolve the merge conflicts.

[apiraino]

based on latest comments, I think I can flip the review flag

@rustbot author

[lcnr]

alright, based on the convo in https://github.com/rust-lang/rust/issues/112651#issuecomment-1838767937 I believe we should remove these projections :sweat_smile: sorry @ouz-a for all the delays and I would love to review this PR if you want to pick it up again