Remove process_result lock since it is an invalid solution

TODO : synchronise process_result in another way

src/threads/thread.h

@@ -37,7 +37,6 @@ struct process_result
   tid_t tid;        /* The tid of the child process. */
   int exit_status;  /* The exit status of the child process. Initially set to
                        -1, then to exit_status when child dies. */
-  struct lock lock; /* Lock to synchronise access to the status and sema. */
   struct semaphore sema; /* Semaphore to signal the parent that the exit_status
                             has been set. */
   struct list_elem elem; /* List element for the parent's children list. */