Rollup merge of #102838 - RalfJung:miri-bootstrap, r=oli-obk

remove cfg(bootstrap) from Miri

Looks like this was forgotten in the bootstrap bump.

r? `@oli-obk`

src/tools/miri/cargo-miri/src/main.rs

@@ -1,4 +1,3 @@
-#![cfg_attr(bootstrap, feature(let_else))]
 #![allow(clippy::useless_format, clippy::derive_partial_eq_without_eq, rustc::internal)]
 
 #[macro_use]

src/tools/miri/src/lib.rs

@@ -10,7 +10,6 @@
 #![feature(is_some_and)]
 #![feature(nonzero_ops)]
 #![feature(local_key_cell_methods)]
-#![cfg_attr(bootstrap, feature(let_else))]
 // Configure clippy and other lints
 #![allow(
     clippy::collapsible_else_if,