JobHistory Table has around 50 million rows,How can we delete this if dont have a HistoryDB option

TRUNCATE TABLE usually does a pretty good job with removing stuff.

But why not simply set config parameter Common\ProcessState\JobHistory\LifeTime to a reasonably value and deactivate Common\ProcessState\JobHistory\IsToExport? Or leave this active but set Common\ProcessState\ExportPolicy to NONE?

 

Actually this parameter is set,but not sure why it is not working

Do you want to preserve the history or not? that's the question here.. if you're not interested you can unconfigure the property model so you're not storing this and don't end up with 50 million records again, then truncate the table to avoid huge rollback transactions and losing temp space.