ASoC: Drop unneeded declaration of removed wm8731 SPI write function

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/sound/soc/codecs/wm8731.c b/sound/soc/codecs/wm8731.c
index 2b980cc..d3fd4f2 100644
--- a/sound/soc/codecs/wm8731.c
+++ b/sound/soc/codecs/wm8731.c
@@ -40,9 +40,6 @@
 	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