Minor cleanups from CR

Co-authored-by: Tulir Asokan <tulir@maunium.net>

database/message.go

@@ -143,7 +143,6 @@ type Message struct {
 	db  *Database
 	log log.Logger
 
-	ID        int
 	Chat      PortalKey
 	JID       types.MessageID
 	MXID      id.EventID

database/upgrades/2022-03-15-prioritized-backfill.go

@@ -23,8 +23,8 @@ func init() {
 				user_mxid           TEXT,
 				type                INTEGER NOT NULL,
 				priority            INTEGER NOT NULL,
-				portal_jid          VARCHAR(255),
-				portal_receiver     VARCHAR(255),
+				portal_jid          TEXT,
+				portal_receiver     TEXT,
 				time_start          TIMESTAMP,
 				time_end            TIMESTAMP,
 				max_batch_events    INTEGER NOT NULL,