kbuild: deb-pkg: do not search for 'scripts' directory under arch/

The 'scripts' directory was searched under arch/${SRCARCH} to copy
arch/ia64/scripts, but commit cf8e8658 ("arch: Remove Itanium
(IA-64) architecture") removed arch/ia64/ entirely.

There is another 'scripts' directory in arch/um/, but this script
is never executed with SRCARCH=um because UML does not support the
linux-headers package.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>