sched: remove skip_sg

This function was used to steer placements to allowed groups.
With earlier improvements on jumping to the correct start_cpu
this usefulness of this function is reduced.

Remove it and move the remaining useful functionality to
find_best_target().

In particular
* skip_sg would skip groups with all cores isolated. This
  would rarely be the situation in practice and moreover the
  loop that scans through cpus in the group would skip isolated
  cpus.
* skip_sg would skip groups once target is found. This can be
  easily moved in to the loop that traverses groups.
* skip_sg checks for situations where a task is confined to
  a cluster and forces us to visit that cluster. This is not
  required anymore as we traverse all the groups.

Change-Id: Ia85b18e44af72b90adacf4bb84b497147263c343
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
[clingutla@codeaurora.org: Fixed trivial merge conflicts]
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>