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/objset.c

@@ -411,8 +411,6 @@ STATIC mp_obj_t set_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
         return set_issubset(lhs, rhs);
     case MP_BINARY_OP_MORE_EQUAL:
         return set_issuperset(lhs, rhs);
-    case MP_BINARY_OP_NOT_EQUAL:
-        return MP_BOOL(set_equal(lhs, rhs) == mp_const_false);
     case MP_BINARY_OP_IN:
     {
         mp_obj_set_t *o = lhs;