remove some calls to INHERIT and de_fault

remove some calls to INHERIT and de_fault

This removes a few unnecessary calls to INHERIT and de_fault:

* to_doc is only used when a target is registered
* to_magic is only used when a target is pushed and not useful for
  current_target.
* to_open and to_close are only ever called using a specific
  target_ops object; there is no need to de_fault them.

2014-02-19  Tom Tromey  <tromey@redhat.com>

	* target.c (update_current_target): Do not INHERIT to_doc or
	to_magic.  Do not de_fault to_open or to_close.