staging: lustre: remove hpdd-discuss list from TODO file

It's a closed mailing list, what's the use of cc:ing patches there if
they never go through.  So remove it from the TODO file.

A real MAINTAINERS entry is probably a good thing for the Lustre code,
maybe someday someone will send a patch to do that...

Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: Oleg Drokin <oleg.drokin@intel.com>
Cc: hpdd-discuss <hpdd-discuss@lists.01.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>