ASoC: Drop unneeded declaration of removed wm8731 SPI write function

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/codecs/wm8731.c

@@ -40,9 +40,6 @@ struct wm8731_priv {
 	unsigned int sysclk;
 };
 
-#ifdef CONFIG_SPI_MASTER
-static int wm8731_spi_write(struct spi_device *spi, const char *data, int len);
-#endif
 
 /*
  * wm8731 register cache