Remove make_cleanup_unpush_target

Remove make_cleanup_unpush_target

This removes make_cleanup_unpush_target, replacing it with a
unique_ptr.  This may seem odd, because the object in question is not
actually freed, but unique_ptr provided the necessary functionality.

Tested by the buildbot.

gdb/ChangeLog
2018-03-30  Tom Tromey  <tom@tromey.com>

	* utils.h (make_cleanup_unpush_target): Remove.
	* inf-ptrace.c (struct target_unpusher): New.
	(target_unpush_up) New typedef.
	(inf_ptrace_create_inferior, inf_ptrace_attach): Use
	target_unpush_up.
	* utils.c (do_unpush_target, make_cleanup_unpush_target): Remove.