Rollup merge of #109238 - spastorino:new-rpitit-12, r=compiler-errors

Fix generics mismatch errors for RPITITs on -Zlower-impl-trait-in-trait-to-assoc-ty

This PR stops reporting errors due to different count of generics on the new synthesized associated types for RPITITs. Those were already reported when we compare the function on the triat with the function on the impl.

r? ``@compiler-errors``

Author: matthiaskrgr