prd: Remove support for discard

Prior to this patch PRD responded to discard requests by zeroing the
discarded LBAs.  Really the intention of discard is to let the storage
know that you no longer require a sector, and forcibly zeroing it but
keeping it statically allocated didn't make much sense.

Instead, just say that we don't support discard.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>