When inheriting from existing content type we inherit all fields.
Some fields are some time not necessary so we need to delete theses fields from our Content Type.
This is a sample code of how to remove the unwanted fields.
So the solution is using the < removefieldref > tag and use the GUID of the unwanted field for the ID param.
To get the GUID of the field ,you can access the 12 folder and go to template/features and then to the parent Content type. then open the Schema.xml of the parent content type and find the specific GUID.
Hope this will be useful