linux(4): Drop a weird comment from linux_set_syscall_retval on amd64

I agree, it would be great to avoid PCB_FULL_IRET, however we should
follow Linux system call ABI.

Reviewed by:		emaste
Differential Revision:	https://reviews.freebsd.org/D40152
MFC after:		1 month

sys/amd64/linux/linux_sysvec.c

@@ -192,9 +192,6 @@ linux_set_syscall_retval(struct thread *td, int error)
 	 * and %r11 values are not preserved across the syscall.
 	 * Require full context restore to get all registers except
 	 * those two restored at return to usermode.
-	 *
-	 * XXX: Would be great to be able to avoid PCB_FULL_IRET
-	 *      for the error == 0 case.
 	 */
 	set_pcb_flags(td->td_pcb, PCB_FULL_IRET);
 }