Fix Unmanaged.fromOpaque() usage in class Thread (#542)

Motivation:

As discussed with Andrew Trick on the Swift Forums, the additional call
to assumingMemoryBound(to:) is redundant here. Unmanaged.fromOpaque
takes a raw pointer, and p is just that.

See https://forums.swift.org/t/withunsafetypepunnedpointer/14540/9
for the discussion.

Modifications:

Deleted an unnecessary call to assumingMemoryBound(to:). With this
change, we follow the intended usage of Unmanaged more closely.

Result:

No material change. We save one redundant UnsafeRawPointer ->
UnsafePointer<ThreadBox> -> UnsafeRawPointer conversion.

Sources/NIO/Thread.swift

@@ -89,7 +89,7 @@ final class Thread {
         let box = ThreadBox(tuple)
         let res = pthread_create(&pt, nil, { p in
             // Cast to UnsafeMutableRawPointer? and force unwrap to make the same code work on macOS and Linux.
-            let b = Unmanaged<ThreadBox>.fromOpaque((p as UnsafeMutableRawPointer?)!.assumingMemoryBound(to: ThreadBox.self)).takeRetainedValue()
+            let b = Unmanaged<ThreadBox>.fromOpaque((p as UnsafeMutableRawPointer?)!).takeRetainedValue()
 
             let body = b.value.body
             let name = b.value.name