kconfig: remove always false ifeq ($(KBUILD_DEFCONFIG,) conditional

With the following two commits applied, all the arch Makefiles
define KBUILD_DEFCONFIG.

- Commit f3e20ad6 ("s390: move arch/s390/defconfig to
  arch/s390/configs/defconfig")

- Commit 986a1376

 ("alpha: move arch/alpha/defconfig to
  arch/alpha/configs/defconfig")

The first conditional in the defconfig rule is always false.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>