Rollup merge of #137516 - RalfJung:rustc_const_unstable-cleanup, r=Amanieu

remove some unnecessary rustc_const_unstable

If the function is anyway unstable, it doesn't need to be `rustc_const_unstable`.

`copy_from_slice` turns out to not do anything const-unstable itself, we just haven't stably committed to it being available in const yet. See [here](https://rustc-dev-guide.rust-lang.org/stability.html?highlight=rustc_const_stable_indirect) for more details on the `rustc_const_stable_indirect` attribute.

Author: tgross35