Remove references to StaticMutex which got removed a while ago

src/librustc_mir/diagnostics.rs

@@ -1281,9 +1281,7 @@ const F: &'static C = &D; // error
 ```
 
 This is because cell types do operations that are not thread-safe. Due to this,
-they don't implement Sync and thus can't be placed in statics. In this
-case, `StaticMutex` would work just fine, but it isn't stable yet:
-https://doc.rust-lang.org/nightly/std/sync/struct.StaticMutex.html
+they don't implement Sync and thus can't be placed in statics.
 
 However, if you still wish to use these types, you can achieve this by an unsafe
 wrapper:

src/libstd/sync/once.rs

@@ -31,12 +31,10 @@
 // initialization closure panics, the Once enters a "poisoned" state which means
 // that all future calls will immediately panic as well.
 //
-// So to implement this, one might first reach for a `StaticMutex`, but those
-// unfortunately need to be deallocated (e.g. call `destroy()`) to free memory
-// on all OSes (some of the BSDs allocate memory for mutexes). It also gets a
-// lot harder with poisoning to figure out when the mutex needs to be
-// deallocated because it's not after the closure finishes, but after the first
-// successful closure finishes.
+// So to implement this, one might first reach for a `Mutex`, but those cannot
+// be put into a `static`. It also gets a lot harder with poisoning to figure
+// out when the mutex needs to be deallocated because it's not after the closure
+// finishes, but after the first successful closure finishes.
 //
 // All in all, this is instead implemented with atomics and lock-free
 // operations! Whee! Each `Once` has one word of atomic state, and this state is

src/libstd/sys/unix/mutex.rs

@@ -49,9 +49,6 @@ impl Mutex {
         // references, we instead create the mutex with type
         // PTHREAD_MUTEX_NORMAL which is guaranteed to deadlock if we try to
         // re-lock it from the same thread, thus avoiding undefined behavior.
-        //
-        // We can't do anything for StaticMutex, but that type is deprecated
-        // anyways.
         let mut attr: libc::pthread_mutexattr_t = mem::uninitialized();
         let r = libc::pthread_mutexattr_init(&mut attr);
         debug_assert_eq!(r, 0);