I have a custom form which is listed as a master form which I cannot delete, I do not want this form how can I delete it ?

Although customisable, there's no option to delete or inactivate the master form, info528.

You might also want to designate your new form as a default template. This is to make sure you're that the master form isn't used in your transactions. Just click the drop-down list beside the new template and select Make default.

More details about this feature are discussed here: Customise Your Invoices, Quotes And Sales Receipts In QuickBooks Online. 

I'll send a feedback and a feature request about being able to remove the master form in QBO. Our engineers might consider adding this option in the future updates. 

I'll jump right back if you have other questions with customising sales templates.