sched/tune: Remove redundant checks for NULL css

The check for NULL css is redundant as upper layers are already
making sure that css cannot be NULL. Remove this check. It helps
to silence static analysis errors as well.

Change-Id: I64585ff8cceb307904e20ff788e52eb05c000e1f
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>