drm/fbdev-helper: Remove drm_fb_helper_output_poll_changed()

The function is unused. Remove it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240812083000.337744-8-tzimmermann@suse.de