Revert "Re-export core::ffi types from std::ffi"

This reverts commit 9aed829fe6.

Fixes https://github.com/rust-lang/rust/issues/96435 , a regression
in crates doing `use std::ffi::*;` and `use std::os::raw::*;`.

We can re-add this re-export once the `core::ffi` types
are stable, and thus the `std::os::raw` types can become re-exports as
well, which will avoid the conflict. (Type aliases to the same type
still conflict, but re-exports of the same type don't.)

library/std/src/lib.rs

@@ -307,7 +307,6 @@
 // Only for re-exporting:
 #![feature(assert_matches)]
 #![feature(async_iterator)]
-#![feature(c_size_t)]
 #![feature(c_variadic)]
 #![feature(cfg_accessible)]
 #![feature(cfg_eval)]