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

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

This reverts commit c0849c1d9c17ba96a37b67363b5551c065e9f50d 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: https://github.com/openwrt/openwrt/issues/9297#issuecomment-1049719381
Signed-off-by: Petr Štetiar <ynezz@true.cz>