You could remove the snapshot records from the database manually using:
lxd sql global 'select instances.name as instance_name, instances_snapshots.id as snapshot_id, instances_snapshots.name as snapshot_name from instances_snapshots join instances on instances.id = instances_snapshots.instance_id order by instance_name, snapshot_id'
+---------------+-------------+---------------+
| instance_name | snapshot_id | snapshot_name |
+---------------+-------------+---------------+
| c1 | 415 | snap0 |
| c1 | 416 | snap1 |
| c1 | 417 | snap2 |
| c1 | 418 | snap3 |
| c1 | 419 | snap4 |
| c1 | 420 | snap5 |
| c1 | 421 | snap6 |
| c1 | 422 | snap7 |
| c1 | 423 | snap8 |
| c1 | 424 | snap9 |
| c1 | 425 | snap10 |
+---------------+-------------+---------------+
and for the associated storage volume snapshot records:
lxd sql global 'select storage_volumes.name as instance_name, storage_volumes_snapshots.id as instance_volume_snapshot_id, storage_volumes_snapshots.name as snapshot_name from storage_volumes_snapshots join storage_volumes on storage_volumes.id = storage_volumes_snapshots.storage_volume_id where storage_volumes.type = 0 order by instance_name, instance_volume_snapshot_id'
+---------------+-----------------------------+-----------------+
| instance_name | instance_volume_snapshot_id | snapshot_name |
+---------------+-----------------------------+---------------+
| c1 | 9472 | snap0 |
| c1 | 9473 | snap1 |
| c1 | 9474 | snap2 |
| c1 | 9475 | snap3 |
| c1 | 9476 | snap4 |
| c1 | 9477 | snap5 |
| c1 | 9478 | snap6 |
| c1 | 9479 | snap7 |
| c1 | 9480 | snap8 |
| c1 | 9481 | snap9 |
| c1 | 9482 | snap10 |
+---------------+-----------------------------+---------------+
Identifying the rows you can remove and then doing:
lxd sql global 'delete from instances_snapshots where id = <snapshot_id>'
lxd sql global 'delete from storage_volumes_snapshots where id = <instance_volume_snapshot_id>'