arch/x86: remove redundant null checks before kmem_cache_destroy

Remove redundant null checks before calling kmem_cache_destroy.

Found with make coccicheck M=arch/x86/kvm on linux-next tag
next-20170929.
Signed-off-by: Tim Hansen <devtimhansen@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -5463,9 +5463,7 @@ static struct shrinker mmu_shrinker = {
 
 static void mmu_destroy_caches(void)
 {
-	if (pte_list_desc_cache)
 	kmem_cache_destroy(pte_list_desc_cache);
-	if (mmu_page_header_cache)
 	kmem_cache_destroy(mmu_page_header_cache);
 }