Stop bubbling out hidden types from the eval obligation queries
r? `@compiler-errors`
I don't know why these were added, but they are not needed anymore. The relevant test is unaffected and I didn't see anything interesting in logging that would have justified it.
This PR has no effect on the new solver behaviour of cf2dff2b1e/tests/ui/impl-trait/issue-99642.rs (which is overflow) and cf2dff2b1e/tests/ui/impl-trait/issue-99642-2.rs (which is "unstable certainty ICE")