m68k/atari: Remove obsolete keyboard_tasklet scheduling

If CONFIG_VT=n:

arch/m68k/atari/built-in.o: In function `atari_keyboard_interrupt':
atakeyb.c:(.text+0x1846): undefined reference to `keyboard_tasklet'
atakeyb.c:(.text+0x1852): undefined reference to `keyboard_tasklet'

I think the keyboard_tasklet scheduling is no longer needed, as I
believe it's handled by drivers/tty/vt/keyboard.c based on events
received from the input subsystem. So just remove it.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Michael Schmitz <schmitz@debian.org>