Remove declarations of is_running/is_stopped/is_exited

The recent commit 08036331 ("Per-inferior thread list, thread
ranges/iterators, down with ALL_THREADS, etc.") removed the
definitions of is_running/is_stopped/is_exited but missed removing the
declarations.

gdb/ChangeLog:
2018-11-23  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (enum thread_state): Move comments here.
	(is_running, is_stopped, is_exited): Remove declarations.