materialize PublicChannels without triggers (#9424)

Creating triggers requires SUPERUSER privileges, and is especially
painful on RDS. Pivot to maintaining this denormalized table in code.

store/sqlstore/supplier.go

@@ -157,15 +157,6 @@ func NewSqlSupplier(settings model.SqlSettings, metrics einterfaces.MetricsInter
 		os.Exit(EXIT_CREATE_TABLE)
 	}
 
-	// This store's triggers should exist before the migration is run to ensure the
-	// corresponding tables stay in sync. Whether or not a trigger should be created before
-	// or after a migration is likely to be decided on a case-by-case basis.
-	if err := supplier.oldStores.channel.(*SqlChannelStoreExperimental).CreateTriggersIfNotExists(); err != nil {
-		mlog.Critical("Error creating triggers", mlog.Err(err))
-		time.Sleep(time.Second)
-		os.Exit(EXIT_GENERIC_FAILURE)
-	}
-
 	UpgradeDatabase(supplier)
 
 	supplier.oldStores.team.(*SqlTeamStore).CreateIndexesIfNotExists()