m68k/mac: Remove obsolete comment

According to The Guide To Mac Family Hardware, this is the correct way
to disable the VBL interrupt. The confusing comment here doesn't add any
value so remove it.

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