Remove outdated function transmute_for_stage0

The function was a workaround for bootstrapping that isn't required
anymore and just degrades hashmap performance, as it doesn't get inlined
cross-crate and turns a no-op into a call.
auto merge of #6640 : dotdash/rust/hash_perf, r=bstrie

The function was a workaround for bootstrapping that isn't required
anymore and just degrades hashmap performance, as it doesn't get inlined
cross-crate and turns a no-op into a call.