sched/fair: do not use boosted margin for prefer_high_cap case

For prefer_high_cap case, it will start from mid/max cpu already,
so there is no need to use boosted margin for task placement.

Bug: 160082718
Test: tasks scheduled as expected
Signed-off-by: Rick Yiu <rickyiu@google.com>
Change-Id: I4df27b1e468484f5d9aedfa57ee444f397a8da81