Remove make_cleanup_free_so

Remove make_cleanup_free_so

make_cleanup_free_so is used in a single spot.  This patch introduces
a unique pointer wrapper for struct so_list, and changes this spot to
use it.

ChangeLog
2017-08-03  Tom Tromey  <tom@tromey.com>

	* utils.h (make_cleanup_free_so): Remove.
	* utils.c (do_free_so, make_cleanup_free_so): Remove.
	* solist.h (struct so_deleter): New.
	(so_list_up): New typedef.
	* solib-svr4.c (svr4_read_so_list): Use so_list_up.