staging: gasket: sysfs: remove legacy_device field

This field is only ever checked, never actually set, and looks to be
left-over from some old interface of some sort.  As it's not being used
at all here, and is just adding to the complexity, delete it.

Cc: Rob Springer <rspringer@google.com>
Cc: John Joseph <jnjoseph@google.com>
Cc: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>