system/skeleton: remove /etc/hostname

We already provide BR2_TARGET_GENERIC_HOSTNAME to set /etc/hostname,
so a default /etc/hostname file isn't needed.

More importantly, if the user has explictly set BR2_TARGET_GENERIC_HOSTNAME
to the empty string, we would still end up with a /etc/hostname containing
'buildroot' which is unlikely to be what the user wanted.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>