m68k: mac: Remove unused rbv_set_video_bpp()

In examining rbv_set_video_bpp()'s improper usage of a non-explictly
signed char type, it turns out that the function is actually unused.
So remove it.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20221026130051.1577286-1-Jason@zx2c4.com


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>