graph/db+sqldb: improve performance of chan update sql migration

This commit simplifies insertChanEdgePolicyMig. Much of the logic can be
removed given that this method is only used in the context of the graph
SQL migration.

This should improve the performance of the migration quite a lot since
it removes the extra GetChannelAndNodesBySCID call.

sqldb/sqlc/queries/graph.sql

@@ -869,11 +869,15 @@ WHERE c.scid = @scid
    ─────────────────────────────────────────────
 */
 
--- name: InsertChanPolicyExtraType :exec
+-- name: UpsertChanPolicyExtraType :exec
 INSERT INTO graph_channel_policy_extra_types (
     channel_policy_id, type, value
 )
-VALUES ($1, $2, $3);
+VALUES ($1, $2, $3)
+ON CONFLICT (channel_policy_id, type)
+    -- If a conflict occurs on channel_policy_id and type, then we update the
+    -- value.
+    DO UPDATE SET value = EXCLUDED.value;
 
 -- name: GetChannelPolicyExtraTypesBatch :many
 SELECT