zram: kill unused zram_get_num_devices()

(cherry pick from commit 0f0e3ba346c8d8d2cb409b157df79805931a1c2c)

Now there's no caller of zram_get_num_devices(), so kill it.
And change zram_devices to static because it's only used in zram_drv.c.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Bug: 24810447
Change-Id: I3b72d6512bcbf5ba055b02fce68a833e28134ded