Revert "scripts/diffconfig.sh: ensure config/conf is built"

This reverts commit c0849c1 as it seems
to introduce regression in config.buildinfo files produced at least by
buildbots:

 $ curl -s https://downloads.openwrt.org/snapshots/targets/ath79/generic/config.buildinfo | head -2
 make[3]: Entering directory '/builder/shared-workdir/build'
 make[3]: Leaving directory '/builder/shared-workdir/build'

References: openwrt/openwrt#9297 (comment)
Signed-off-by: Petr Štetiar <ynezz@true.cz>

scripts/diffconfig.sh

@@ -8,7 +8,6 @@ grep '^CONFIG_DEVEL=y' .config >> tmp/.diffconfig.head
 grep '^CONFIG_TOOLCHAINOPTS=y' .config >> tmp/.diffconfig.head
 grep '^CONFIG_BUSYBOX_CUSTOM=y' .config >> tmp/.diffconfig.head
 grep '^CONFIG_TARGET_PER_DEVICE_ROOTFS=y' .config >> tmp/.diffconfig.head
-make -s ./scripts/config/conf
 ./scripts/config/conf --defconfig=tmp/.diffconfig.head -w tmp/.diffconfig.stage1 Config.in >/dev/null
 ./scripts/kconfig.pl '>+' tmp/.diffconfig.stage1 .config >> tmp/.diffconfig.head
 ./scripts/config/conf --defconfig=tmp/.diffconfig.head -w tmp/.diffconfig.stage2 Config.in >/dev/null