FIXUP: sched/tune: add fixes missing from a previous patch

The previous patch:

  e7ce26f0 - FIXUP: sched/tune: fix accounting for runnable tasks

squashed together patches of a series to fix SchedTune's accounting
issues. However, in the consolidation and cleanup of the series to merge
in the Android Common Kernel, we somehow missed a couple of important
changes:
1) the schedtune_exit function is not more required, because e7ce26f0
   fixes accounting of exiting tasks in a different way
2) the schedtune_initialized flag was not set at the end of
   scheddtune_init_cgroup() thus failing to enabled SchedTune at boot.

This patch thus is to be considered an integration of e7ce26f0.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>