ANDROID: sched/tune: remove unused variable

This value was added in:
   e71c4255 ANDROID: sched/tune: Add support for negative boost values

and it's likely coming from a conflict resolution or a merge of some
previous patches used to track negative boost values.

Change-Id: I3b63e99db9232eb6117a561aa61c527986ade8e4
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>