slub: Remove CONFIG_SMP defined check

As CONFIG_SMP is one of dependencies of CONFIG_SLUB_CPU_PARTIAL, so if
CONFIG_SLUB_CPU_PARTIAL is defined then CONFIG_SMP must be defined,
no need to check CONFIG_SMP definition here.

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>