Auto merge of #87768 - rust-lang:core-features-cleanup, r=dtolnay

Core features cleanup

This sorts and categorizes the `#![features]` in `core` and removes unused ones.

This is part of #87766

The following feature attributes were unnecessary and are removed:

```diff
// Library features:
-#![feature(bool_to_option)]
-#![feature(char_indices_offset)]
-#![feature(pin_deref_mut)]
-#![feature(str_split_as_str)]
-#![feature(str_split_inclusive_as_str)]

// Language features:
-#![feature(arbitrary_self_types)]
-#![feature(custom_inner_attributes)]
-#![feature(nll)]
```

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