Hi Krishna,
you can follow Bernard's instructions to delete that field. But do you really want to delete Activity field? If you allow only one activity (e.g. 03 - Display) then it's same as not having field there. You can also use DUMMY instead of field in ABAP to ignore this field. Why would you keep that field there? Keeping this field there will allow you to introduce more granular authorization checks when your custom apps becomes more complex. And that is what usually happens. You start with a simple app but business usually requires more functionality in future.
Cheers