Remove unique constraint from relations table.

This aligns with the removal of the unique constraint in the assets
table.

This will likely improve performance and avoid some deadlocks faced in
production as shown below from the innodb status.

*** (2) TRANSACTION:
TRANSACTION 13224836, ACTIVE 2 sec inserting
mysql tables in use 1, locked 1
908 lock struct(s), heap size 106616, 943 row lock(s), undo log entries 1951
MySQL thread id 4405, OS thread handle 140199291565824, query id 676173554 10.236.102.30 svc-criteokb Update
INSERT INTO relations (id, from_id, to_id, type) VALUES (?, ?, ?, ?)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 41 page no 34334 n bits 528 index unique_relation of table `criteokb`.`relations` trx id 13224836 lock_mode X locks rec but not gap
Record lock, heap no 455 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 8; hex e74f2e13ed67611d; asc  O.  ga ;;
 1: len 8; hex 7a9f7cd5e9f8c4c0; asc z |     ;;
 2: len 3; hex 686173; asc has;;
 3: len 8; hex 74af2536c993d1f2; asc t %6    ;;

Author: clems4ever

internal/database/mariadb.go

@@ -83,8 +83,6 @@ func (m *MariaDB) InitializeSchema() error {
 			CONSTRAINT fk_from FOREIGN KEY (from_id) REFERENCES assets (id),
 			CONSTRAINT fk_to FOREIGN KEY (to_id) REFERENCES assets (id),
 
-			UNIQUE unique_relation (from_id, to_id, type),
-
 			INDEX full_relation_type_from_to_idx (type, from_id, to_id),
 			INDEX full_relation_type_to_from_idx (type, to_id, from_id),
 			INDEX full_relation_from_type_to_idx (from_id, type, to_id),