ASoC: audio-graph-card: remove endpoint bidirectional check

DTC commit df536831d02c ("checks: add graph binding checks")
is checking endpoint bidirectional, and it is upstreamed to linux by
commit 50aafd60 ("scripts/dtc: Update to upstream version
v1.4.6-21-g84e414b0b5bc").
Let's remove own bidirectional check

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