Open an existing bootable image file, and the Properties Bar will display "Bootable Image".
Choose menu item {"Action" --> "Boot" --> "Delete Boot information"} .
Click 
to save the file.
Delete Boot Information from Bootable Image
Open an existing bootable image file, and the Properties Bar will display "Bootable Image".
Choose menu item {"Action" --> "Boot" --> "Delete Boot information"} .
Click
to save the file.