Use std::set in mi-main.c

Use std::set in mi-main.c

Change a couple of spots in mi-main.c to use std::set.  This
simplifies the code and removes some cleanups.

gdb/ChangeLog
2017-09-29  Tom Tromey  <tom@tromey.com>

	* mi/mi-main.c (struct print_one_inferior_data) <inferiors>: Now a
	'std::set *'.
	(print_one_inferior): Update.
	(free_vector_of_ints): Remove.
	(list_available_thread_groups): Change "ids" to std::set.
	(mi_cmd_list_thread_groups): Update.
	(struct collect_cores_data) <core>: Now a std::set.
	(collect_cores): Update.
	(unique): Remove.
	(print_one_inferior): Update.