ASoC: Remove per card platform list

The platform_dev_list was added in commit f0fba2ad

 ("ASoC: multi-component -
ASoC Multi-Component Support") and while platforms are added and remove from
that list it is otherwise unused. This patch removes it again.

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