Remove a special case for XEN, which is erronous and makes vfork(2)

behaviour to differ from the documented, only on XEN.  If there are
any issues with XEN pmap left, they should be fixed in pmap.

MFC after:	2 weeks

sys/kern/kern_fork.c

@@ -150,11 +150,7 @@ sys_vfork(struct thread *td, struct vfork_args *uap)
 	int error, flags;
 	struct proc *p2;
 
-#ifdef XEN
-	flags = RFFDG | RFPROC; /* validate that this is still an issue */
-#else
 	flags = RFFDG | RFPROC | RFPPWAIT | RFMEM;
-#endif		
 	error = fork1(td, flags, 0, &p2, NULL, 0);
 	if (error == 0) {
 		td->td_retval[0] = p2->p_pid;