Revert "backlight/fbcon: Add FB_EVENT_CONBLANK"

This reverts commit 994efacd.

The justification is that if hw blanking fails (i.e. fbops->fb_blank)
fails, then we still want to shut down the backlight. Which is exactly
_not_ what fb_blank() does and so rather inconsistent if we end up
with different behaviour between fbcon and direct fbdev usage. Given
that the entire notifier maze is getting in the way anyway I figured
it's simplest to revert this not well justified commit.

v2: Add static inline to the dummy version.

Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Yisheng Xie <ysxie@foxmail.com>
Cc: linux-fbdev@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-25-daniel.vetter@ffwll.ch