Revert "SGX mutex is movable"

This reverts commit 30b82e0f96579d9f897c4e2a780af82662d89772.

library/std/src/sys/sgx/mutex.rs

@@ -8,7 +8,7 @@ pub struct Mutex {
     inner: SpinMutex<WaitVariable<bool>>,
 }
 
-pub type MovableMutex = Mutex;
+pub type MovableMutex = Box<Mutex>;
 
 // Implementation according to “Operating Systems: Three Easy Pieces”, chapter 28
 impl Mutex {