fbdev: Remove FB_DEFAULT_SYS_OPS

Remove the initializer macro FB_DEFAULT_SYS_OPS and its helper macro
__FB_DEFAULT_SYS_OPS_MMAP. There are no users.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Cc: Helge Deller <deller@gmx.de> (maintainer:FRAMEBUFFER LAYER)
Link: https://patchwork.freedesktop.org/patch/msgid/20230707083422.18691-12-tzimmermann@suse.de