netconsole: do not depend on experimental

Nowadays, most distributions enable netconsole by default,
including RHEL, Fedora, Debian, Arch, Opensuse. And
we don't have any bug reports about it. So I think there
is no need to mark it as experimental any more.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: WANG Cong <amwang@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>