Removing a field from an object type does not change the historical information for existing package lines using the given object type. Any values for the deleted field remain in the package lines table in the column specified in the field definition.
To remove a field permanently from an object type:
From the Workbench shortcut bar, select Deployment Mgmt > Object Types.
The Object Type Workbench opens.
Open an object type.
The Object Type window opens.
From the Fields tab, select a field, and then click Remove.