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.
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