Replace finish_thread_state_cleanup with a RAII class

Replace finish_thread_state_cleanup with a RAII class

gdb/ChangeLog:
2018-04-10  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (finish_thread_state_cleanup): Delete declaration.
	(scoped_finish_thread_state): New class.
	* infcmd.c (run_command_1): Use it instead of finish_thread_state
	cleanup.
	* infrun.c (proceed, prepare_for_detach, wait_for_inferior)
	(fetch_inferior_event, normal_stop): Likewise.
	* thread.c (finish_thread_state_cleanup): Delete.