ASoC: soc-core: remove unused num_dai_links

ALSA SoC is counting card->dai_link_list user,
but no-one is using it.
Let's remove it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>