Remove code intended to put threads to sleep.

Remove code intended to put threads to sleep.

Actually, threads would "go to sleep" either by blocking on a heap
allocation, or by noticing `scm_i_thread_go_to_sleep' is set when
running `SCM_TICK', which limits the applicability of this technique
(e.g., it was not appropriate for the shared string code).

* libguile/threads.c (scm_i_thread_go_to_sleep, scm_i_thread_put_to_sleep,
  scm_i_thread_invalidate_freelists, scm_i_thread_wake_up,
  scm_i_thread_sleep_for_gc): Remove.

* libguile/threads.h (scm_i_thread_go_to_sleep, scm_i_thread_put_to_sleep,
  scm_i_thread_invalidate_freelists, scm_i_thread_wake_up,
  scm_i_thread_sleep_for_gc): Remove declarations.
  (SCM_THREAD_SWITCHING_CODE): Do nothing.