Drop Chance/SimpleScheduler Implementation

Drop Chance/SimpleScheduler Implementation

This patch removes the implementation of ChanceScheduler and SimpleScheduler
as previous changes have made sure they are internally replaced by
FilterScheduler.

The "max_gigabytes" config option is deprecated and will leave it like that
for one more release before we can remove it.

DocImpact: "ChanceScheduler and SimpleScheduler have been deprecated and
their implementation have been removed from Cinder."

Implement bp: deprecate-chance-and-simple-schedulers

Change-Id: Ifb1cb25e3bb4cdf26fa3283336b83fce5c97141e