s/Get(Master|Replica)X/Get\1/g (#29520)

Drop the legacy `X` suffix from `GetMasterX` and `GetReplicaX`. The
presence of the suffix suggests there's a `non-X` version: but in fact
we migrated these away a long time ago, so remove the cognitive
overhead.

As an aside, this additionally helps avoid trip up LLMs that interpret
this as "something to fix".

server/channels/store/sqlstore/migrate.go

@@ -141,7 +141,7 @@ func (ss *SqlStore) initMorph(dryRun bool) (*morph.Morph, error) {
 		}
 		defer db.Close()
 	case model.DatabaseDriverPostgres:
-		driver, err = ps.WithInstance(ss.GetMasterX().DB.DB)
+		driver, err = ps.WithInstance(ss.GetMaster().DB.DB)
 	default:
 		err = fmt.Errorf("unsupported database type %s for migration", ss.DriverName())
 	}