greybus: operation: remove gb_operation_destroy

Remove legacy interface to "destroy" operations, which is now just a
wrapper for gb_operation_put.

The old interface name hides the fact that all operations are refcounted
and may live on even after having "destroyed" them.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>