lpfc: Delete unnecessary checks before the function call "mempool_destroy"

The mempool_destroy() function tests whether its argument is NULL
and then returns immediately. Thus the test around the calls is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: James Smart <james.smart@avagotech.com>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Reviewed-by: Sebastian Herbszt <herbszt@gmx.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>