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/querier.go

@@ -87,7 +87,6 @@ type Querier interface {
 	HighestSCID(ctx context.Context, version int16) ([]byte, error)
 	InsertAMPSubInvoice(ctx context.Context, arg InsertAMPSubInvoiceParams) error
 	InsertAMPSubInvoiceHTLC(ctx context.Context, arg InsertAMPSubInvoiceHTLCParams) error
-	InsertChanPolicyExtraType(ctx context.Context, arg InsertChanPolicyExtraTypeParams) error
 	InsertChannelFeature(ctx context.Context, arg InsertChannelFeatureParams) error
 	// NOTE: This query is only meant to be used by the graph SQL migration since
 	// for that migration, in order to be retry-safe, we don't want to error out if
@@ -141,6 +140,7 @@ type Querier interface {
 	UpdateInvoiceHTLCs(ctx context.Context, arg UpdateInvoiceHTLCsParams) error
 	UpdateInvoiceState(ctx context.Context, arg UpdateInvoiceStateParams) (sql.Result, error)
 	UpsertAMPSubInvoice(ctx context.Context, arg UpsertAMPSubInvoiceParams) (sql.Result, error)
+	UpsertChanPolicyExtraType(ctx context.Context, arg UpsertChanPolicyExtraTypeParams) error
 	UpsertChannelExtraType(ctx context.Context, arg UpsertChannelExtraTypeParams) error
 	UpsertEdgePolicy(ctx context.Context, arg UpsertEdgePolicyParams) (int64, error)
 	UpsertNode(ctx context.Context, arg UpsertNodeParams) (int64, error)