Rollup merge of #120037 - compiler-errors:remove-next-root-var-helper, r=lcnr

Remove `next_root_ty_var`

Uhh we seem to not have any test that relies on this anymore. Maybe due to the way we changed alias-relate or whatever.

Removing this hack helper fn because #119106 is the general solution.

r? lcnr

Author: matthiaskrgr