Garbage collect thread continuations

Garbage collect thread continuations

Nothing uses thread continuations anymore.

(inferior continuations are still used by the attach command.)

gdb/ChangeLog:
2015-09-09  Pedro Alves  <palves@redhat.com>

	* continuations.c (add_continuation, restore_thread_cleanup)
	(do_all_continuations_ptid, do_all_continuations_thread_callback)
	(do_all_continuations_thread, do_all_continuations)
	(discard_all_continuations_thread_callback)
	(discard_all_continuations_thread, discard_all_continuations)
	(add_intermediate_continuation)
	(do_all_intermediate_continuations_thread_callback)
	(do_all_intermediate_continuations_thread)
	(do_all_intermediate_continuations)
	(discard_all_intermediate_continuations_thread_callback)
	(discard_all_intermediate_continuations_thread)
	(discard_all_intermediate_continuations): Delete.
	* continuations.h (add_continuation, do_all_continuations)
	(do_all_continuations_thread, discard_all_continuations)
	(discard_all_continuations_thread, add_intermediate_continuation)
	(do_all_intermediate_continuations)
	(do_all_intermediate_continuations_thread)
	(discard_all_intermediate_continuations)
	(discard_all_intermediate_continuations_thread): Delete
	declarations.
	* event-top.c (stdin_event_handler): Delete references to
	continuations.
	* gdbthread.h (struct thread_info): Delete continuations and
	intermediate_continuations fields.
	* inf-loop.c (inferior_event_handler): Remove references to
	continuations.
	* infrun.c (infrun_thread_stop_requested_callback): Remove
	references to continuations.
	* target.h (enum inferior_event_type) <INF_EXEC_CONTINUE>: Delete.
	* thread.c: Don't include "continuations.h".
	(clear_thread_inferior_resources): Remove references to
	continuations.