parisc: remove ARCH_DEFCONFIG

Commit 2a86f661 ("kbuild: use KBUILD_DEFCONFIG as the fallback for
DEFCONFIG_LIST") removed ARCH_DEFCONFIG because it does not make much
sense.

In the same development cycle, Commit ededa081

 ("parisc: Fix
defconfig selection") added ARCH_DEFCONFIG for parisc.

Please use KBUILD_DEFCONFIG in arch/*/Makefile for defconfig selection.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Helge Deller <deller@gmx.de>