Use std::list for remote_notif_state::notif_queue

Use std::list for remote_notif_state::notif_queue

This changes remote_notif_state::notif_queue to be a std::list and
updates all the uses.

gdb/ChangeLog
2019-04-19  Tom Tromey  <tom@tromey.com>

	* remote.c (remote_target): Use delete.
	* remote-notif.h: Include <list>, not "common/queue.h".
	(notif_client_p): Remove typedef.
	(remote_notif_state): Add constructor, destructor, initializer.
	<notif_queue>: Now a std::list.
	(remote_notif_state_xfree): Don't declare.
	* remote-notif.c (remote_notif_process, handle_notification)
	(remote_notif_state_allocate): Update.
	(~remote_notif_state): Rename from remote_notif_state_xfree.