kernel/idle: Clean up and refactoring / remove TICKLESS_IDLE_THRESH

While I'm in the idle code, let's clean this loop up.  It was a really
bad #ifdef hell:

* Remove the CONFIG_TICKLESS_IDLE_THRESH logic (and the kconfig),
  which never did anything but needlessly increase latency.

* Move the needed timeout logic from the main loop into
  pm_save_idle(), which eliminates the special case for
  !SYS_CLOCK_EXISTS.

Behavior (modulo that one kconfig) should be completely unchanged, and
now the inner part of the idle loop looks like:

    while (true) {
        (void) arch_irq_lock();

        if (IS_ENABLED(CONFIG_PM)) {
            pm_save_idle();
        } else {
            k_cpu_idle();
        }
    }

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>