refactor(core): remove #![feature(nonnull_slice_from_raw_parts)]

The `nonnull_slice_from_raw_parts` feature was stabilized by
[rust-lang/rust#97506][1].

[1]: rust-lang/rust#97506

Author: yvt

src/r3_core/src/lib.rs

@@ -5,7 +5,6 @@
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(maybe_uninit_array_assume_init)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
-#![feature(nonnull_slice_from_raw_parts)]
 #![feature(const_intoiterator_identity)]
 #![feature(type_changing_struct_update)]
 #![feature(maybe_uninit_uninit_array)]