Rollup merge of #113427 - compiler-errors:no-variances-of-rpitit-gat, r=spastorino

Remove `variances_of` on RPITIT GATs, remove its one use-case

It doesn't make sense to implement variances on a GAT anyways, since we don't relate GATs with variance:

https://github.com/rust-lang/rust/blob/85bf07972a1041b9e25393b803d0e006bec3eaaf/compiler/rustc_middle/src/ty/relate.rs#L569-L579

r? ``@spastorino``

Author: compiler-errors