Rollup merge of #133872 - compiler-errors:simplify-gat-check, r=oli-obk

No need to create placeholders for GAT args in confirm_object_candidate

We no longer need this logic to add placeholders for GAT args since with the removal of the `gat_extended` feature gate (rust-lang/rust#133768) we no longer allow GATs in dyn trait anyways.

r? oli-obk