vhost/scsi: Remove unused but set variable

Remove the unused but set variable se_tpg in vhost_scsi_nexus_cb() to
fix the following GCC warning when building with 'W=1':

  drivers/vhost/scsi.c:1752:26: warning: variable ‘se_tpg’ set but not used

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>