Remove a cleanup from target-descriptions.c

Remove a cleanup from target-descriptions.c

This removes a cleanup from target-descriptions.c, by changing it to
use a unique_ptr instead.  Note that a deletion adapter is used, even
though target_desc is allocated with new, to avoid moving target_desc
to target-descriptions.h.

gdb/ChangeLog
2019-01-02  Tom Tromey  <tom@tromey.com>

	* xml-tdesc.c (xml_cache): Hold a target_desc_up.
	(tdesc_parse_xml): Remove cleanups.
	* target-descriptions.h (make_cleanup_free_target_description):
	Don't declare.
	(target_desc_deleter): New struct.
	(target_desc_up): New typedef.
	* target-descriptions.c (target_desc_deleter::operator()): Rename
	from free_target_description.
	(make_cleanup_free_target_description): Remove.