This should de-link the parent form and the sub form. This also means any data you had put through on the subform will also no longer be linked to the parent form. If you need the data to be linked, I recommend downloading the data before de-linking the forms.
As forms can act as sub forms for multiple parent forms, you will need to ensure that you have completed the above steps for every parent form the sub form is linekd to.
Once you have de-linked the forms, the process to delete the sub form is the same as deleting any other form.