Best way to drop a field from a content type
There are two api functions to delete fields:


field_delete_field To delete the entire field (all instances of of the field)
field_delete_instance To delete only an instance of a field - which is how it's attached to an entity.


If you don't want to reuse the field, it might be a bit cleaner to delete the entire field instead of only delete the instance of it on the content type in question. Only you can be the judge of that.