m68k/atari: atakeyb.c - Remove some unused functions

Remove some functions that are not used anywhere:
atari_kbd_leds() ikbd_exec() ikbd_mem_read() ikbd_mem_write()
ikbd_clock_get() ikbd_clock_set() ikbd_pause() ikbd_resume()

This was partially found by using a static code analysis program called
cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>