ASoC: Drop empty platform remove functions

Merge series from Uwe Kleine-König <u.kleine-koenig@pengutronix.de>:

Hello,

this patch series removes all platform remove functions that only return
zero below sound/soc. There is no reason to have these, as the only
caller is platform core code doing:

        if (drv->remove) {
                int ret = drv->remove(dev);

                ...
        }

(in platform_remove()) and so having no remove function is both
equivalent and simpler.