crush: remove 'class rm' command

crush: remove 'class rm' command

The current version is broken. E.g., it should only remove a class
which is never referenced by any device.

Since we now create new classes automatically, we shall automatically
recycle dead classes too. So this command is definitely unuseful.
(Actually it is weird that we keep 'class rm' without keeping the
 corresponding 'class create' command).

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>