s390/kfence: Split kfence pool into 4k mappings in arch_kfence_init_pool()

Since commit d08d4e7cd6 ("s390/mm: use full 4KB page for 2KB PTE"),
there is no longer any reason to avoid splitting the kfence pool into
4k mappings in arch_kfence_init_pool(). Remove the architecture-specific
kfence_split_mapping().

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/mm/init.c

@@ -175,7 +175,6 @@ void __init mem_init(void)
         high_memory = (void *) __va(max_low_pfn * PAGE_SIZE);
 
 	pv_init();
-	kfence_split_mapping();
 
 	/* this will put all low memory onto the freelists */
 	memblock_free_all();