staging/lustre/obdclass: remove unused file md_attrs.c

It's only used by server side luster for on-disk structure swabbing.

And also remove forward declarations for its functions from
md_object.h and lustre_idl.h

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>