fbdev/core: Remove empty internal helpers from fb_logo.c

Remove the two empty helpers for the case the CONFIG_FB_LOGO_EXTRA
has not been set. They are internal functions and only called once.
Providing empty replacements seems like overkill. Instead protect
the call sites with a test for CONFIG_FB_LOGO_EXTRA.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230907085408.9354-7-tzimmermann@suse.de