Use a single ReentrantMutex implementation on all platforms.

library/std/src/sys/unsupported/locks/mutex.rs

@@ -36,26 +36,3 @@ impl Mutex {
     #[inline]
     pub unsafe fn destroy(&self) {}
 }
-
-// All empty stubs because this platform does not yet support threads, so lock
-// acquisition always succeeds.
-pub struct ReentrantMutex {}
-
-impl ReentrantMutex {
-    pub const unsafe fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex {}
-    }
-
-    pub unsafe fn init(&self) {}
-
-    pub unsafe fn lock(&self) {}
-
-    #[inline]
-    pub unsafe fn try_lock(&self) -> bool {
-        true
-    }
-
-    pub unsafe fn unlock(&self) {}
-
-    pub unsafe fn destroy(&self) {}
-}