m68k: remove unused adb.h

The asm-m68k/adb.h header is unused. Some definitions are wrong and the rest
are duplicated in linux/adb.h. Remove it.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>