ASoC: codecs: remove hidden prompt

The Kconfig symbol SND_SOC_OF_SIMPLE got removed in commit
f0fba2ad ("ASoC: multi-component - ASoC
Multi-Component Support"). But that commit missed one instance. Remove
it now, together with the prompt it has effectively hidden ever since.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>