staging/lustre: KEY_DEFAULT/MAX_COOKIESIZE key is unused, remove them

Also remove the lmv and mdc infrastructure to query these keys.
In fact this whole "unlink cookies" thing becomes irrelevant
with newer servers, but since we still retain 2.[123] servers
commpatibility, we cannot completely remove all traces of it yet.

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