Remove "sys isn't exported yet" phrase

The oldest occurence is from 9e224c2bf1,
which is from the pre-1.0 days. In the years since then, std::sys still
hasn't been exported, and the last attempt was met with strong criticism:
https://github.com/rust-lang/rust/pull/97151

Thus, removing the "yet" part makes a lot of sense.

library/std/src/sys/unix/locks/pthread_mutex.rs

@@ -16,7 +16,6 @@ pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}
 
-#[allow(dead_code)] // sys isn't exported yet
 impl Mutex {
     pub const fn new() -> Mutex {
         // Might be moved to a different address, so it is better to avoid