sched: eliminate sched_migration_fixup knob

Kill unused scheduler knob sched_migration_fixup.  With this change
scheduler always adjusts CPU's busy time during migration.

Change-Id: I5d59e89d5cc0f2c705c40036cd7b47f5d3f89e58
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>