mm/percpu: remove unused PERCPU_DYNAMIC_EARLY_SLOTS

Since commit 40064aec ("percpu: replace area map allocator with
bitmap"), there's no place to use PERCPU_DYNAMIC_EARLY_SLOTS. So
clean it up.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>