fbdev/atafb: Remove unused extern variables

Remove 6 unused extern variables to reduce confusion. It is worth
mentioning that lib/fonts/font_8x8.c and lib/fonts/font_8x16.c also
declare `fontdata_8x8` and `fontdata_8x16` respectively, and this file
has nothing to do with them.

Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/cb5bb49a33ff54fef41e719ee9d301a6a73c5f9c.1603788512.git.yepeilin.cs@gmail.com
Link: https://patchwork.freedesktop.org/patch/msgid/20201028105647.1210161-1-yepeilin.cs@gmail.com