create table test(a int, b int);

insert into test values(1,2),(1,3),(2,1),(2,5),(3,1),(3,4);

select * from test;
+------+------+
| a    | b    |
+------+------+
|    1 |    2 |
|    1 |    3 |
|    2 |    1 |
|    2 |    5 |
|    3 |    1 |
|    3 |    4 |
+------+------+

delete test from test join (select t2.c3 from
(select if(a<=b,a,b) c1, if(a<=b,b,a) c2, concat(a,'_',b) c3 from test) t1 join
(select if(a<=b,a,b) c1, if(a<=b,b,a) c2, concat(a,'_',b) c3 from test) t2
on t1.c1=t2.c1 and t1.c2=t2.c2 and t1.c3<t2.c3) t on concat(a,'_',b)=t.c3;

select * from test;
+------+------+
| a    | b    |
+------+------+
|    1 |    2 |
|    1 |    3 |
|    2 |    5 |
|    3 |    4 |
+------+------+