Revert "sched: Make struct sched_statistics independent of fair sched class"

This reverts commit c3b9f955 which is
commit ceeadb83 upstream.

It breaks the Android kernel abi and odds are, is not needed for Android
systems at this point in time.  If it needs to be applied, it can come
back in an abi-safe way.

Bug: 161946584
Change-Id: Ie160ea068245566066bb712943a720927a8f6347
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>