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>

scripts/Makefile.kasan

 # SPDX-License-Identifier: GPL-2.0
-ifdef CONFIG_KASAN
 CFLAGS_KASAN_NOSANITIZE := -fno-builtin
 KASAN_SHADOW_OFFSET ?= $(CONFIG_KASAN_SHADOW_OFFSET)
-endif
 
 ifdef CONFIG_KASAN_GENERIC