[Tracking issue] removing `unsafe` implementations copied from `core`

[VictorKoenders]

• maybe_uninit_slice Tracking issue for #![feature(maybe_uninit_slice)] rust-lang/rust#63569
• maybe_uninit_uninit_array (no tracking issue)
• maybe_uninit_array_assume_init Tracking Issue for maybe_uninit_array_assume_init rust-lang/rust#80908

rust-lang/rust#80908 is closed now