scsi: lpfc: remove NULL check before some freeing functions

As dma_pool_destroy and mempool_destroy functions has NULL check. We may
not need NULL check before calling them.

Fix below warnings reported by coccicheck
./drivers/scsi/lpfc/lpfc_mem.c:252:2-18: WARNING: NULL check before some
freeing functions is not needed.
./drivers/scsi/lpfc/lpfc_mem.c:255:2-18: WARNING: NULL check before some
freeing functions is not needed.
./drivers/scsi/lpfc/lpfc_mem.c:258:2-18: WARNING: NULL check before some
freeing functions is not needed.
./drivers/scsi/lpfc/lpfc_mem.c:261:2-18: WARNING: NULL check before some
freeing functions is not needed.
./drivers/scsi/lpfc/lpfc_mem.c:265:2-18: WARNING: NULL check before some
freeing functions is not needed.
./drivers/scsi/lpfc/lpfc_mem.c:269:2-17: WARNING: NULL check before some
freeing functions is not needed.

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>