Remove cleanup from procfs.c

Remove cleanup from procfs.c

This removes the last remaining cleanup from procfs.c, replacing it
with a unique_ptr specialization.

gdb/ChangeLog
2018-09-13  Tom Tromey  <tom@tromey.com>

	* procfs.c (struct procinfo_deleter): New.
	(procinfo_up): New typedef.
	(do_destroy_procinfo_cleanup): Remove.
	(procfs_target::info_proc): Use procinfo_up.  Remove cleanups.