Remove Miri special-case

Author: zachs18

std/src/sys/pal/common/exit_guard.rs

@@ -1,13 +1,5 @@
 cfg_if::cfg_if! {
-    if #[cfg(miri)] {
-        /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
-        ///
-        /// This mitigation is not necessary when running under Miri, so this function does nothing
-        /// when running under Miri.
-        pub(crate) fn unique_thread_exit() {
-            // Mitigation not required on Miri, where `exit` is thread-safe.
-        }
-    } else if #[cfg(target_os = "linux")] {
+    if #[cfg(target_os = "linux")] {
         /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
         ///
         /// On UNIX-like platforms (where `libc::exit` may not be thread-safe), ensure that only one