spi: dw: Remove unused dw_spi_cleanup function

The dw_spi_cleanup() function was removed by commit c63f5da0
"spi: dw: Don't call kfree for memory allocated by devm_kzalloc".

commit ec37e8e1 "spi: dw: migrate to generic queue infrastructure" added
dw_spi_cleanup() but never use it. So now I got below build warning:

  CC [M]  drivers/spi/spi-dw.o
drivers/spi/spi-dw.c:609:13: warning: 'dw_spi_cleanup' defined but not used [-Wunused-function]

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mark Brown <broonie@linaro.org>