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

The `generic_associated_types` feature was stabilized by
[rust-lang/rust#96709][1].

[1]: rust-lang/rust#96709

Author: yvt

src/r3_core/src/lib.rs

@@ -11,7 +11,6 @@
 #![feature(maybe_uninit_uninit_array)]
 #![feature(const_precise_live_drops)]
 #![feature(const_raw_ptr_comparison)]
-#![feature(generic_associated_types)]
 #![feature(associated_type_bounds)]
 #![feature(const_slice_first_last)]
 #![feature(cfg_target_has_atomic)] // `#[cfg(target_has_atomic_load_store)]`