Revert "[ConstraintSystem] Make it possible to infer subtype bindings through argument conversions"

[DougGregor]

Reverts #30006. It caused a regression that we'd like to address before re-landing:

struct X {
  var cgf: CGFloat
}

func test(x: X?) {
  let _ = (x?.cgf ?? 0) <= 0.5
}

[DougGregor]

@swift-ci please smoke test

[xedin]

That’s unfortunate, I will try to take a look asap...

[xedin]

I'm going to close this one in favor of #30283 where I've fixed failing test-case and added a new one for rdar://problem/60185506.

[DougGregor]

Okay!