skeleton: remove unused /etc/issue

/etc/issue is managed with BR2_TARGET_GENERIC_ISSUE. In case
BR2_TARGET_GENERIC_ISSUE is set (which is default), /etc/issue
is overwritten. In case BR2_TARGET_GENERIC_ISSUE is not set,
we don't want /etc/issue exist.

Finaly, remove /etc/issue from skeleton.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: "Maxime Hadjinlian" <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>