x86/boot: Remove unused function atou()

I can't find any sign of atou() having been used. Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240913005753.1392431-1-linux@treblig.org