libnftables: drop check for nf_sock in nft_ctx_free()

libnftables: drop check for nf_sock in nft_ctx_free()

The "nft_ctx" API does not provide a way to change or reconnect the
netlink socket. And none of the users would rely on that.

Also note that nft_ctx_new() initializes nf_sock via
nft_mnl_socket_open(), which panics of the socket could not be
initialized.

This means, the check is unnecessary and needlessly confusing. Drop it.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>