tcm_fc: Remove cmd->cdb data member

It's used only for debug output. Debug output may want to make use of
fcp->fc_cdb directly.

Signed-off-by: Andy Grover <agrover@redhat.com>
Cc: Kiran Patil <kiran.patil@intel.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>