dummycon: Stop exporting dummycon_[un]register_output_notifier

Now that we only allow FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER when
fbdev+fbcon are builtin exporting these is no longer necessary.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>