Eliminate target_ops::to_xclose

In the multi-target branch, I found no need for the target_close vs
target_xclose distinction.  Heap-allocated targets simply delete
themselves in their target_close implementation, while
singleton/static targets don't.

The target_ops C++ification patches will add more commentary around
target_ops's destructor, but there's no destructor yet...

gdb/ChangeLog:
2018-05-02  Pedro Alves  <palves@redhat.com>

	* bfd-target.c (target_bfd_xclose): Rename to ...
	(target_bfd_close): ... this.
	(target_bfd_reopen): Adjust.
	* target.c (target_close): Remove references to to_xclose.
	* target.h (target_ops::to_xclose): Delete.
	(target_ops::to_close): Update comments.