ASoC: Remove unused extern declarations for sh4_hac_dai and sh7760_soc_platform

Both sh4_hac_dai and sh7760_soc_platform are changed to static
by multi-component patch and they are not used in sh7760-ac97.c now.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>