py: Remove useless implementations of NOT_EQUAL in binary_op's.

I'm pretty sure these are never reached, since NOT_EQUAL is always
converted into EQUAL in mp_binary_op.  No one should call
type.binary_op directly, they should always go through mp_binary_op
(or mp_obj_is_equal).

py/objlist.c

@@ -138,8 +138,6 @@ STATIC mp_obj_t list_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
         case MP_BINARY_OP_MORE:
         case MP_BINARY_OP_MORE_EQUAL:
             return MP_BOOL(list_cmp_helper(op, lhs, rhs));
-        case MP_BINARY_OP_NOT_EQUAL:
-            return MP_BOOL(!list_cmp_helper(MP_BINARY_OP_EQUAL, lhs, rhs));
 
         default:
             // op not supported