kbuild: remove redundant CONFIG_KASAN check from scripts/Makefile.kasan

Since commit e0fe0bbe ("kbuild: include scripts/Makefile.* only
when relevant CONFIG is enabled"), this file is included only when
CONFIG_KASAN=y.

This ifdef is redundant.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Marco Elver <elver@google.com>