UBI: drop null test before destroy functions

Remove unneeded NULL test.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@@ expression x; @@
-if (x != NULL)
  \(kmem_cache_destroy\|mempool_destroy\|dma_pool_destroy\)(x);
// </smpl>

CRs-Fixed: 975289
Change-Id: I6a10dba4c17e8d9983b8dee633ba6c7b2785f7c6
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: f9a113d65fa56d8e8e662e37ec2fbeac0d52aa01
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>