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/testlib/helper.go

@@ -222,7 +222,7 @@ func (h *MainHelper) PreloadMigrations() {
 			panic(fmt.Errorf("cannot read file: %v", err))
 		}
 	}
-	handle := h.SQLStore.GetMasterX()
+	handle := h.SQLStore.GetMaster()
 	_, err = handle.Exec(string(buf))
 	if err != nil {
 		panic(errors.Wrap(err, "Error preloading migrations. Check if you have &multiStatements=true in your DSN if you are using MySQL. Or perhaps the schema changed? If yes, then update the warmup files accordingly"))