ASoC: dapm: Remove always true path source/sink checks

A path has always a valid source and a valid sink otherwise we wouldn't add
it in the first place. Hence all tests that check if sink/source is non NULL
always evaluate to true and can be removed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>