mm/slab: remove unused slab_early_init

'slab_early_init' was introduced by 'commit e0a42726
("[PATCH] mm/slab.c: fix early init assumption")', this
flag was used to prevent off-slab caches being created
so early during bootup.

The only user of 'slab_early_init' was removed in 'commit
 3217fd9b ("mm/slab: make criteria for off slab
determination robust and simple")'.

Signed-off-by: Gou Hao <gouhao@uniontech.com>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>