minor ravenscar-thread cleanup

minor ravenscar-thread cleanup

The use of the ravenscar_open static global is unnecessary.
This patch removes it. It also removes some routines that
were created in association with the management of this global.

gdb/ChangeLog:

        * ravenscar-thread.c (ravenscar_is_open): Delete.
        (ravenscar_initialize): Move most of the code from here...
        (ravenscar_inferior_created): ... to there.  Invert logic.
        (ravenscar_close): Delete.
        (init_ravenscar_thread_ops): Remove setting of
        ravenscar_ops.to_close.